refactor(static): load the image editor on first use (#6074)

galleryEditor.js and its js/editor/ graph are 54 modules / 576 KB, and
gallery.js imported them statically. Every page load paid for the whole
image editor even though most sessions never open the Edit tab: 54 of the
173 JS files on a cold load, and 576 KB of the decoded JS, were for a
panel that was never displayed.

Add a small panel-loader registry (static/js/panels.js) that imports a
panel's module on first use and memoises the promise, so a double-click
cannot start two loads and a failed load can still be retried. Convert
the image editor to it, and route the two existing dynamic imports in
chat.js and chatRenderer.js through the same entry so all three call
sites share one module instance instead of two.

closeEditor() and isEditorOpen() stay synchronous: if the module was
never loaded there is no edit session to close and none can be open.

The service worker keeps precaching the editor, in a separate
PANEL_PRECACHE list, so the panel stays available offline even though
index.html no longer loads it. The two lists now serve different
purposes and the header comment says so.

static/js/gallery.js

@@ -3,7 +3,7 @@
  */
 
 import uiModule from './ui.js';
-import { openEditor, closeEditor, isEditorOpen } from './galleryEditor.js?v=20260708match1';
+import { loadPanel } from './panels.js';
 import spinnerModule from './spinner.js';
 import { makeWindowDraggable } from './windowDrag.js';
 import { bindMenuDismiss, dismissOrRemove } from './escMenuStack.js';
@@ -15,6 +15,54 @@ const API_BASE = window.location.origin;
 let _open = false;
 let _galleryResizeHandler = null;
 
+// ── Image editor, loaded on first use ──
+// galleryEditor.js plus everything under js/editor/ is 54 modules / 576 KB.
+// It used to be a static import here, so every page load paid for it even
+// though most sessions never touch the Edit tab. The wrappers below keep the
+// three call shapes the rest of this file already uses.
+//
+// closeEditor() and isEditorOpen() stay synchronous on purpose: if the module
+// was never loaded there is no edit session to close, and none can be open.
+let _editorMod = null;
+let _editorLoading = false;
+
+async function _loadEditor() {
+  _editorLoading = true;
+  try {
+    _editorMod = await loadPanel('editor');
+    return _editorMod;
+  } finally {
+    _editorLoading = false;
+  }
+}
+
+async function openEditor(...args) {
+  let mod = _editorMod;
+  if (!mod) {
+    try {
+      mod = await _loadEditor();
+    } catch (e) {
+      // Previously unreachable — a static import either loaded or the whole
+      // page failed. Now it can fail on its own (offline before the panel was
+      // ever cached), so say so instead of doing nothing.
+      console.error('[gallery] image editor failed to load', e);
+      uiModule?.showError?.('Failed to load the image editor');
+      return;
+    }
+  }
+  return mod.openEditor(...args);
+}
+
+function closeEditor(...args) {
+  return _editorMod ? _editorMod.closeEditor(...args) : undefined;
+}
+
+// True while the module is still in flight as well — the gallery-close paths
+// use this to refuse to tear the container down under an edit that is opening.
+function isEditorOpen() {
+  return _editorLoading || (_editorMod ? _editorMod.isEditorOpen() : false);
+}
+
 // Auto-refresh gallery when new image is generated
 window.addEventListener('gallery-refresh', (e) => {
   if (e?.detail?.source === 'chat-upload' && _sort !== 'recent') {