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.

tests/plugin-list.spec.ts

@@ -2,11 +2,11 @@
  * Built-in plugin catalog data-integrity tests (both files: the TAB
  * catalog in plugins-tabs.ts and the FILE-PREVIEWER catalog in
  * plugins-viewers.ts). Every entry must be installable as data — a unique
- * id (npm package name), a GitHub URL, a non-empty localized description
- * (string or () => string), and an install script that mentions the dsh
- * plugin CLI. The catalogs are the discovery surface of the two "add
- * plugin" modals; a malformed entry would break the install flow, so the
- * shape is pinned here.
+ * id (npm package name), a GitHub URL, a non-empty localized name and
+ * description (string or () => string), and an install script that mentions
+ * the dsh plugin CLI. The catalogs are the discovery surface of the two
+ * "add plugin" modals; a malformed entry would break the install flow, so
+ * the shape is pinned here.
  */
 import { describe, expect, it } from 'vitest'
 import { builtinTabPlugins } from '../src/client/plugins-tabs.ts'
@@ -42,9 +42,11 @@ describe('builtin plugin catalogs', () => {
         }
       })
 
-      it('every entry has a name, a GitHub URL, and an install script', () => {
+      it('every entry has a localized name, a GitHub URL, and an install script', () => {
         for (const entry of list) {
-          expect(entry.name.length).toBeGreaterThan(0)
+          // Names went through the same i18n-friendly union as descriptions
+          // (string or () => string), so resolve before asserting.
+          expect(textOf(entry.name).length).toBeGreaterThan(0)
           expect(entry.url.startsWith('https://github.com/')).toBe(true)
           expect(entry.install.length).toBeGreaterThan(0)
           expect(entry.install).toContain('dsh plugin')