Merge the file-icon theme shell from #594 (feat/file-icon-theme)

Brings in the theme plumbing around #531's API — the `fileIconTheme` pref
thread, the settings row, the 20 dictionaries' keys and the theme loader's
test — so the working tree is a complete, green starting point before the
colored implementation is replaced.

Conflicts (three; resolved by taking this line's side where it had moved on):

- src/client/service.ts: the import block keeps `baseName, extOf` (the
  `isNarrowWidth` import the theme branch needed belongs to its own
  fresh-session seeding, which main has since rewritten) and the feature-list
  doc comment keeps main's "v0.19.0 REMOVED 'floatWindows'" note while
  adopting the sharper `fileIcons` wording (matched by exts / names /
  folderNames).
- tests/plugin-shape.spec.ts and tests/prefs.spec.ts: main DELETED the store
  seeding tests these files used to hold and rewrote the schema assertions
  (the `SIDEBAR_PREFS_DEFAULTS` destructuring), so this line's side wins —
  the theme branch's re-added blocks describe behaviour main no longer has.

Next commit removes `fileIconTheme` again along with the 563-entry dataset and
its lazy chunk: DSH 0.1.5-rc.2 exports the official `FileTypeIcon` artwork, so
the plugin renders the host's icons instead of shipping its own color table.

src/client/index.tsx

@@ -24,6 +24,7 @@ import { registerBottomToggle } from './sidebar/bottom-toggle.tsx'
 import { createNativeSurface } from './native/surface.ts'
 import { registerLinkInterception } from './link-intercept.ts'
 import { registerImeGuard } from './ime-guard.ts'
+import { createFileIconThemeLoader } from './file-icon-theme.ts'
 import { registerSettingsNavIcon } from './settings-nav-icon.ts'
 import { loadBootDecision } from './prefs.ts'
 import { SideCardSection } from './SideCardSection.tsx'
@@ -188,6 +189,21 @@ export function apply(ctx: Context): void {
     () => registerBuiltins(ctx, service, { terminalTitle: () => terminalTitle }),
     'dsh-better-sidebar: register built-in tabs and viewers',
   )
+  // Optional colored file-icon theme: the `fileIconTheme` pref decides whether
+  // the lazy `file-icons` chunk is fetched and its dataset registered through
+  // `registerFileIcon`. The loader reacts to prefs writes and is disposed with
+  // the fiber (HMR-safe); 'builtin' (the default) keeps the startup path free
+  // of icon data.
+  ctx.effect(() => {
+    const loader = createFileIconThemeLoader(service)
+    const sync = (): void => { loader.set(sidebarStore.getPrefs().fileIconTheme) }
+    sync()
+    const unsubscribe = service.subscribeState(sync)
+    return () => {
+      unsubscribe()
+      loader.dispose()
+    }
+  }, 'dsh-better-sidebar: colored file-icon theme')
   // A failure anywhere in the client lifecycle must never take the app down
   // silently: log with the plugin prefix and pin a visible diagnostic strip
   // to the page so a blank panel is never the only symptom. This strip is