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.

tests/consumer-types.ts

@@ -132,9 +132,11 @@ service.openFile({ sessionId: 's1', cwd: '/p' }, '/p/a.csv', 'Data')
 const icon: FileIconDescriptor = {
   id: 'my-plugin:icons',
   exts: ['csv', FOLDER_EXT, FOLDER_OPEN_EXT],
+  names: ['package.json'],
+  folderNames: ['node_modules'],
   priority: 5,
-  icon: (path: string, size: number) => {
-    void path; void size
+  icon: (path: string, size: number, open?: boolean) => {
+    void path; void size; void open
     return null
   },
 }
@@ -142,6 +144,7 @@ service.registerFileIcon(icon)
 service.getFileIcons()
 service.matchFileIcon('/p/a.csv')
 service.matchFolderIcon(true)
+service.matchFolderIcon(true, 'node_modules')
 service.fileIcon('/p/a.csv', 14)
 service.folderIcon('/p', true, 14)