MultitabPane: keepAlive mode and tab strip polish

Adds keepAlive: when true, all entry bodies stay mounted via
IndexedStack so switching tabs doesn't tear down their state.
Hosts that own PTY-backed sessions or any long-lived widget
state opt in; callers that want fresh state on each switch use
the default single-body mode.

Polishes the tab strip itself for production use:
- bottom divider so the strip visually anchors to the body below
- Column.crossAxisAlignment.stretch so the strip fills the pane
  width instead of sizing to its content
- close button: replace the text × glyph with the CloseIcon
  painter (clean cross strokes, font-independent)
- two-column tab layout — Expanded title on the left, fixed
  16x16 close button on the right; uniform 12px left padding,
  6px right padding to match the 6px top/bottom breathing room
  around the close button

Two new widget tests cover keepAlive (state preserved across
switches) and default mode (inactive bodies disposed).

Co-Authored-By: Claude <noreply@anthropic.com>

.pql/pql-plan.json

@@ -1,5 +1,5 @@
 {
-  "exported_at": "2026-05-06T12:17:16Z",
+  "exported_at": "2026-05-06T13:20:57Z",
   "decisions": [
     {
       "id": "D-1",
@@ -2073,11 +2073,11 @@
       "type": "task",
       "parent_id": "T-3",
       "title": "secondary Claude pane UI wiring",
-      "status": "ready",
+      "status": "in_progress",
       "priority": "medium",
       "decision_ref": "D-41",
       "created_at": "2026-04-22 14:08:40",
-      "updated_at": "2026-05-03 20:36:31"
+      "updated_at": "2026-05-06 12:20:58"
     },
     {
       "id": "T-25",
@@ -2687,18 +2687,13 @@
       "id": "T-84",
       "type": "task",
       "title": "MultitabPane: drag-to-reorder gesture wiring",
-      "status": "in_progress",
+      "status": "done",
       "priority": "medium",
       "created_at": "2026-05-06 10:16:02",
-      "updated_at": "2026-05-06 10:17:57"
-    }
-  ],
-  "ticket_deps": [
-    {
-      "blocker_id": "T-84",
-      "blocked_id": "T-24"
+      "updated_at": "2026-05-06 12:17:27"
     }
   ],
+  "ticket_deps": null,
   "ticket_labels": null,
   "history": [
     {
@@ -4056,6 +4051,20 @@
       "old_value": "backlog",
       "new_value": "in_progress",
       "changed_at": "2026-05-06 10:17:57"
+    },
+    {
+      "ticket_id": "T-84",
+      "field": "status",
+      "old_value": "in_progress",
+      "new_value": "done",
+      "changed_at": "2026-05-06 12:17:27"
+    },
+    {
+      "ticket_id": "T-24",
+      "field": "status",
+      "old_value": "ready",
+      "new_value": "in_progress",
+      "changed_at": "2026-05-06 12:20:58"
     }
   ]
 }