fix(sidebar): animate the sidebar toggle instead of snapping

The global .hidden utility (display: none) also matches #sidebar.hidden,
so the collapsed sidebar was removed from layout before its width/opacity
transition could run, and opening and closing both snapped in one frame.

Keep the collapsed sidebar rendered and take it out of interaction once
closed: .sidebar.hidden stays display: flex, gains pointer-events: none,
and visibility: hidden that is transitioned so it only flips after the
collapse finishes (keeping the closed sidebar out of the tab order).

On desktop the icon rail now tucks under the sidebar and slides out as it
collapses, so the combined width eases instead of jumping by 48px when
the two swap. On mobile the drawer keeps its width while off so it slides
out as well as in, and switching it to the right edge before opening no
longer animates it across the screen. Transitions are disabled under
prefers-reduced-motion, and the auto-collapse restore measures the chat
width with the transition suppressed.

Re-record the computed-style snapshot baseline for the sidebar and icon
rail.

tests/css_snapshot/baseline.json

@@ -1,5 +1,5 @@
 {
- "digest": "f7e60e4a567c46dc",
+ "digest": "5282a075729cc246",
  "elements": {
   "app-shell": {
    "app-loader": "f04ad5bf6312d53f",
@@ -17,7 +17,7 @@
    "export-dropdown-item": "d90f6f513d06a6f7",
    "export-dropdown-menu": "0793e06a6aae6a7a",
    "hamburger-btn": "34714f6eef3734d8",
-   "icon-rail": "e3f72205260a4a5f",
+   "icon-rail": "c800b4ef07f5f1f0",
    "icon-rail-btn": "54ffb10e056ebf44",
    "incognito-btn": "ee42bd3f6a32576b",
    "input-icon-btn": "3d1c773acd84a825",
@@ -59,7 +59,7 @@
    "settings-row": "de568724268cec6e",
    "settings-select": "a96e8430b0ec6668",
    "settings-sidebar-toggle": "6be9025ab24bc434",
-   "sidebar": "77ec42a646a8db66",
+   "sidebar": "0dbf510da61afa07",
    "sidebar-brand": "d3d7ef4b21514e80",
    "sidebar-new-chat": "567fae30048b7ff3",
    "sidebar-resize-handle": "6607b819ee45409d",
@@ -488,7 +488,7 @@
    ".hwfit-serve-topline .hwfit-serve-runtime-note": "15b3fef4e5ceaffc",
    ".hwfit-serve-topline .hwfit-serve-runtime-text": "57c6822b0d13147d",
    ".hwfit-spec-group .hwfit-spec-method": "5d1c329116272cf1",
-   ".icon-rail": "e3f72205260a4a5f",
+   ".icon-rail": "c800b4ef07f5f1f0",
    ".icon-rail-btn": "a5f207c3be15401e",
    ".incognito-btn": "9833a2e86fab6a94",
    ".incognito-indicator": "b3945c46844c7278",
@@ -617,15 +617,15 @@
    ".settings-sidebar.settings-sidebar-collapsed": "d9d8f25df95d9ad8",
    ".settings-sidebar.settings-sidebar-collapsed .settings-sidebar-content": "608e51170864586a",
    ".settings-system-logs-tab": "d0d90a13ff327ace",
-   ".sidebar": "77ec42a646a8db66",
+   ".sidebar": "0dbf510da61afa07",
    ".sidebar .section": "37fc733d59519a08",
    ".sidebar-brand-title": "2b20f75768b50346",
    ".sidebar-header": "0d58816ca65b7216",
    ".sidebar-inner": "ad5e28fedbfeb5db",
    ".sidebar-notif-dot": "2673e215d0cd84a6",
    ".sidebar-user-bar": "b77372dbdcb069ef",
-   ".sidebar.hidden": "0df301b638e2f93f",
-   ".sidebar.right-side": "2b6bcc7d59f7b6bd",
+   ".sidebar.hidden": "9021ec7e4e052131",
+   ".sidebar.right-side": "bb6ed4427d07657e",
    ".skill-card > .skill-card-header": "e2408e762818d2fb",
    ".skill-card-tags": "a327c85e053e6d3f",
    ".skill-card-textcol": "f1059ed37c120b60",
@@ -686,56 +686,56 @@
  },
  "variants": {
   "app-shell": {
-   "desktop-dark-comfortable": "0c9123117135e362",
-   "desktop-dark-compact": "c694f5d5118ac0e2",
-   "desktop-dark-spacious": "90a67613e8cfdf5d",
-   "desktop-light-comfortable": "17727a9945ccc77b",
-   "desktop-light-compact": "2a887c30d231e357",
-   "desktop-light-spacious": "94b4666734b87625",
-   "laptop-dark-comfortable": "26d334b139b653f2",
-   "laptop-dark-compact": "fea30bad6824357d",
-   "laptop-dark-spacious": "4fdb28deaaaec870",
-   "laptop-light-comfortable": "c7fc986a7e70aac8",
-   "laptop-light-compact": "147645431229ddff",
-   "laptop-light-spacious": "d513c285d30c8289",
-   "phone-dark-comfortable": "551761344bec8d09",
-   "phone-dark-compact": "293d66518c3ec994",
-   "phone-dark-spacious": "551761344bec8d09",
-   "phone-light-comfortable": "cb41acd3f9af3578",
-   "phone-light-compact": "dbef6e9e373e9c10",
-   "phone-light-spacious": "cb41acd3f9af3578",
-   "tablet-dark-comfortable": "0f682621b4ead393",
-   "tablet-dark-compact": "e6f3fece5e716a2c",
-   "tablet-dark-spacious": "0f682621b4ead393",
-   "tablet-light-comfortable": "6e398df9ca69c58b",
-   "tablet-light-compact": "dca75981c8675bfa",
-   "tablet-light-spacious": "6e398df9ca69c58b"
+   "desktop-dark-comfortable": "a401d90436a5ca64",
+   "desktop-dark-compact": "207348542b2e4e0f",
+   "desktop-dark-spacious": "cf93e41d494c2fdc",
+   "desktop-light-comfortable": "e080e0286a12ad19",
+   "desktop-light-compact": "5ecb27fdc079e051",
+   "desktop-light-spacious": "97eeb23477e3ac77",
+   "laptop-dark-comfortable": "c3142729689b9c36",
+   "laptop-dark-compact": "2d0bd3f5e3075c14",
+   "laptop-dark-spacious": "4c9a7ddc3fa236e4",
+   "laptop-light-comfortable": "234d92df9bed6f4c",
+   "laptop-light-compact": "a619f09b8b9e33e5",
+   "laptop-light-spacious": "5dea4413f33a9002",
+   "phone-dark-comfortable": "86f8033e0a23b873",
+   "phone-dark-compact": "2df6fce436ebbaac",
+   "phone-dark-spacious": "86f8033e0a23b873",
+   "phone-light-comfortable": "0067d466b77774f5",
+   "phone-light-compact": "8c82bcc55dd57b63",
+   "phone-light-spacious": "0067d466b77774f5",
+   "tablet-dark-comfortable": "dfeed4c65fea98a1",
+   "tablet-dark-compact": "cbdb84a6edc362bf",
+   "tablet-dark-spacious": "dfeed4c65fea98a1",
+   "tablet-light-comfortable": "7399146b3d437f7f",
+   "tablet-light-compact": "be6bf69a5215b052",
+   "tablet-light-spacious": "7399146b3d437f7f"
   },
   "bench": {
-   "desktop-dark-comfortable": "a9709cb47eec7544",
-   "desktop-dark-compact": "bfab96cb72fd6faf",
-   "desktop-dark-spacious": "25469178b443428a",
-   "desktop-light-comfortable": "2166794d643fae4c",
-   "desktop-light-compact": "81d8ab64407b1d9a",
-   "desktop-light-spacious": "d6d19f33476665af",
-   "laptop-dark-comfortable": "57169e64e1fb5324",
-   "laptop-dark-compact": "6ac903cd8a711bcf",
-   "laptop-dark-spacious": "a47535332e9c5de7",
-   "laptop-light-comfortable": "1e2b381de3556fac",
-   "laptop-light-compact": "6dbfb465121949c1",
-   "laptop-light-spacious": "1b8bfcd4bc4ed214",
-   "phone-dark-comfortable": "67ca3c78e6b9495a",
-   "phone-dark-compact": "ad6f3a9f5c8d81cd",
-   "phone-dark-spacious": "1e6f66e56519ba5d",
-   "phone-light-comfortable": "ef876073cc2dc45e",
-   "phone-light-compact": "2ef6f5dc7a646be9",
-   "phone-light-spacious": "b0d4cd9bd0361966",
-   "tablet-dark-comfortable": "58922240b216e786",
-   "tablet-dark-compact": "3991c22f99a394e0",
-   "tablet-dark-spacious": "9848c3b7debac974",
-   "tablet-light-comfortable": "12dfbad36172de20",
-   "tablet-light-compact": "80d4843e622f1353",
-   "tablet-light-spacious": "c420b8d46c991a44"
+   "desktop-dark-comfortable": "477394268d56dab4",
+   "desktop-dark-compact": "cdb2a994f2347d9a",
+   "desktop-dark-spacious": "44a0286474f509bc",
+   "desktop-light-comfortable": "b4991b72e1629a7e",
+   "desktop-light-compact": "4b31438b6cf29191",
+   "desktop-light-spacious": "6d2bb1699acb8ba1",
+   "laptop-dark-comfortable": "6b8df96c7323bd57",
+   "laptop-dark-compact": "9af052dd3ee47593",
+   "laptop-dark-spacious": "7d031596ee98a11f",
+   "laptop-light-comfortable": "352a8564ecebd3dd",
+   "laptop-light-compact": "c038f51edaf78a6b",
+   "laptop-light-spacious": "232cfb9c427a156d",
+   "phone-dark-comfortable": "1f66ccc3d46e3d59",
+   "phone-dark-compact": "14802a927af47d2a",
+   "phone-dark-spacious": "3b48b91c9f218777",
+   "phone-light-comfortable": "191dd9704ea10245",
+   "phone-light-compact": "2f659a85548fd8d6",
+   "phone-light-spacious": "ece925a707889ec4",
+   "tablet-dark-comfortable": "7075293d10d618b5",
+   "tablet-dark-compact": "792f8efb8de95a25",
+   "tablet-dark-spacious": "b7ec70ad523b5a2e",
+   "tablet-light-comfortable": "bdf9d78bf3cd5ced",
+   "tablet-light-compact": "990bb4d23e0fbac9",
+   "tablet-light-spacious": "53c5142d2fca6fc1"
   },
   "login": {
    "desktop-dark-comfortable": "3cf4809db0f8e119",