fix: UI language reverts to English after a second save in Settings

[yyyzl]

Fixes #104.

Problem

After switching the UI language to Chinese in Settings → Interface and saving, opening another draft-bound section (Embedding / Multimodal / Web Search / Network / Output), editing any field, and clicking Save again silently flips the entire UI back to English and persists language=en to the store. Reproduces 100% on main (v0.4.5).

Root cause

SettingsView has a resync useEffect that rebuilds the whole draft whenever any of the store slices change, and re-reads i18n.language while doing so. handleSave calls multiple zustand setters before it finally calls i18n.changeLanguage at the end — and each await between setters lets React commit, which fires the resync effect mid-save. At that point i18n.language is still the OLD value, so the effect silently rewrites draft.uiLanguage back to the old language.

The first save still works because handleSave closes over its own snapshot of draft (with the new uiLanguage), so if (draft.uiLanguage !== i18n.language) still takes the change-language branch successfully. But after that save:

• i18n.language === "zh" ✅
• but the component's draft.uiLanguage === "en" (clobbered by the mid-save resync)

The next save reads the stale draft.uiLanguage = "en", hits the same if condition with "en" !== "zh", and helpfully calls changeLanguage("en") + saveLanguage("en") — reverting and persisting the language flip the user never asked for.

Fix

One-line change in the resync effect: keep the current draft.uiLanguage via a functional setState instead of re-reading i18n.language. uiLanguage is only ever written by the Interface section's setDraft, never out-of-band, so preserving it across store-driven resyncs is correct.

 useEffect(() => {
-  setDraftState(
+  setDraftState((prev) =>
     initialDraft(
       llmConfig, searchApiConfig, embeddingConfig, multimodalConfig,
       outputLanguage, proxyConfig, maxHistoryMessages,
-      i18n.language,
+      prev.uiLanguage,
     ),
   );
 }, [...]);

Plus a code comment explaining why this is intentional, so the next person doesn't "fix" it back.

Verification

Manual repro on main:

1. en → 中文 in Interface, Save → UI in Chinese ✅, draft.uiLanguage actually became "en" 🐛
2. Open Embedding, edit endpoint, Save → UI flips to English 🐛

After this patch, step 2 keeps the UI in Chinese, and language=zh stays persisted.

The fix doesn't change any other section's behavior:

• LLM section persists per-row independently (doesn't touch the shared draft Save)
• About / Maintenance / Changelog have no draft-bound fields
• Project switch → out-of-band store change → resync still happens, and prev.uiLanguage at that point is the value used to mount the panel (which equals i18n.language at mount time), so behavior is unchanged for that scenario.