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/builtins/tabs.tsx

@@ -8,7 +8,7 @@
  * `browser:<n>` the same way (no quota). The editor IS the files window
  * (the old standalone explorer merged into it).
  */
-import { IconCodeOutline16, IconFolderOpen16, IconNewChatOutline16, IconPanelLeftOutline16, IconThinkOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
+import { IconCodeOutline16, IconFolderOpen16, IconFolderOpenOutline16, IconNewChatOutline16, IconPanelLeftOutline16, IconSparkle16, IconThinkOutline16 } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { Context } from '../../context-types.ts'
 import { allLeaves, isAgentTabId, type SidebarState } from '../state.ts'
 import { t } from '../locales.ts'
@@ -122,6 +122,28 @@ export function builtinTabs(ctx: Context, options: BuiltinTabOptions = {}): read
           key: 'workspaceFence',
           title: () => t('settingsFenceTitle'),
           desc: () => t('settingsFenceDesc'),
+        }, {
+          // The file-icon theme: monochrome built-in glyphs (default) vs the
+          // optional colored brand dataset, which is fetched from the lazy
+          // `file-icons` chunk only when picked (see file-icon-theme.ts).
+          key: 'fileIconTheme',
+          type: 'select',
+          title: () => t('settingsFileIconThemeTitle'),
+          desc: () => t('settingsFileIconThemeDesc'),
+          options: [
+            {
+              value: 'builtin',
+              icon: (size: number) => <IconFolderOpenOutline16 size={size} />,
+              title: () => t('settingsFileIconThemeBuiltin'),
+              desc: () => t('settingsFileIconThemeBuiltinDesc'),
+            },
+            {
+              value: 'colored',
+              icon: (size: number) => <IconSparkle16 size={size} />,
+              title: () => t('settingsFileIconThemeColored'),
+              desc: () => t('settingsFileIconThemeColoredDesc'),
+            },
+          ],
         }],
         render: ({ pluginSettings, updatePluginSetting }) => (
           <OpenWithSettings pluginSettings={pluginSettings} updatePluginSetting={updatePluginSetting} />