perf: move the 19 non-zh/en dictionaries into a lazy locale chunk

The core client bundle statically imported every third-language
dictionary (ja/de/fr/…/zh-HK/MO/TW, ~640KB of source — roughly half
the bundle's weight) for a code path most sessions never enter: the
sidebar's own t() only consults zh/en plus the better-locale override
store, and the dicts' ONLY consumer is that store's register() call.

They now ride the existing lazy-chunk machinery as lib/client-locale.js
(fetched from /sidebar/bundle only when better-locale is actually
installed; a generation counter keeps the async registration moot when
the integration effect re-runs or disposes mid-flight). Until the chunk
lands, t() keeps the zh/en chain; the store's own revision bump on
register re-renders the chrome once the dicts arrive.

Core client.js: 1.46MB → 822KB (-44%). The chunk-boundary key-set
check moves to the existing runtime test (every shipped dict stays
key-set-equal to zh).

tests/bundle-route.spec.ts

@@ -145,6 +145,6 @@ describe('/sidebar/bundle route', () => {
   })
 
   it('exports the chunk allowlist (mirror of src/client/chunk-loader.ts)', () => {
-    expect([...CHUNK_NAMES]).toEqual(['terminal', 'editor', 'mermaid'])
+    expect([...CHUNK_NAMES]).toEqual(['terminal', 'editor', 'mermaid', 'locale'])
   })
 })