feat(i18n): localize plugin catalog names and shell preset desc

Catalog entry names (15 tab + 4 viewer) and the dsh-desktop shell preset
desc were raw hardcoded zh strings rendered straight into the add-plugin
modals and the title-bar scheme dropdown. Mirror the existing desc
pattern exactly: PluginEntry.name / ShellPreset.desc widen to the
i18n-friendly string-or-() =>-string union, the catalogs carry lazy
t('pluginXxxName') / t('presetDshDesktopDesc') lookups, and the
add-plugin modal (render, search filter, aria-labels) plus the scheme
SelectMenu resolve them like every other copy. The plain-string arm of
the union stays as the raw-name fallback, and t() itself never yields
empty text (missing keys render the key), so a dict gap can never blank
a name.

20 new dictionary keys added to zh/en (locales.ts), ja (locales-ja.ts),
and all 18 third-language dictionaries — tests/locales.spec.ts enforces
key-set equality against zh for every shipped dict (precedent: 9cc642b),
so the 'leave other locales-* untouched' shortcut would fail the hard
parity gate; real translations are provided instead.

Spec follow-ups (behavior change: names/descs now render through t()):
- plugin-list.spec.ts: name non-empty assertion resolves the union.
- add-plugin-modal.spec.tsx: sentinel/office name assertions resolve
  from the catalog (en locale in tests) instead of pinning zh literals.
- shell-presets.spec.ts: desc non-empty assertion resolves the union.

src/client/plugins-viewers.ts

@@ -2,8 +2,9 @@
  * The built-in catalog of FILE-PREVIEWER plugins (file-type previewers),
  * shown in the "add preview plugin" modal (Side card settings → 文件预览
  * grid → the dashed card). Adding an entry: append one object here (unique
- * `id` = npm package name, `url` = GitHub repo, `description` =
- * i18n-friendly, `install` = the full shell command pre-filled into the
+ * `id` = npm package name, `name` / `description` = i18n-friendly (add a
+ * `pluginXxxName` / `pluginXxxDesc` key in locales.ts), `url` = GitHub repo,
+ * `install` = the full shell command pre-filled into the
  * install terminal — it starts with `cd ~/.dsh` so the install runs with
  * the DSH home as the working directory). Data integrity is guarded by
  * `tests/plugin-list.spec.ts`.
@@ -15,28 +16,28 @@ import type { PluginEntry } from './plugins-shared.ts'
 export const builtinViewerPlugins: readonly PluginEntry[] = [
   {
     id: '@huanlin/dsh-plugin-better-sidebar-plugin-office',
-    name: 'Office 预览插件',
+    name: () => t('pluginOfficeName'),
     url: 'https://github.com/HuanLinOTO/dsh-plugin-better-sidebar-plugin-office',
     description: () => t('pluginOfficeDesc'),
     install: 'cd ~/.dsh && dsh plugin --profile web add @huanlin/dsh-plugin-better-sidebar-plugin-office',
   },
   {
     id: 'dsh-md-export',
-    name: 'Markdown 导出插件',
+    name: () => t('pluginMdExportName'),
     url: 'https://github.com/AnakinCao/dsh-md-export',
     description: () => t('pluginMdExportDesc'),
     install: 'cd ~/.dsh && dsh plugin --profile web add dsh-md-export',
   },
   {
     id: 'dsh-code-nav',
-    name: '代码预览导航',
+    name: () => t('pluginCodeNavName'),
     url: 'https://github.com/AnakinCao/dsh-code-nav',
     description: () => t('pluginCodeNavDesc'),
     install: 'cd ~/.dsh && dsh plugin --profile web add https://github.com/AnakinCao/dsh-code-nav.git',
   },
   {
     id: 'dsh-video-preview',
-    name: '视频预览插件',
+    name: () => t('pluginVideoPreviewName'),
     url: 'https://github.com/zemul/dsh-video-preview',
     description: () => t('pluginVideoPreviewDesc'),
     install: 'cd ~/.dsh && dsh plugin --profile web add dsh-video-preview',