feat(i18n): complete bilingual UI support (en/zh-CN) — closes #567

Addressed in 9bb3302: settingsViewConstants.ts no longer evaluates i18n.t() at module load. DICTATION_MODELS now stores labelKey/noteKey, composer presets and section labels store key strings only; each consumer (SettingsDictationSection, SettingsComposerSection, SettingsView) renders through t() from useTranslation, so these labels now update immediately when the Display language is switched. Verified: 1032/1032 tests pass, typecheck clean.

Both review points addressed in 9682f3d:

1. v4 plural forms — added _one/_other variants for all 11 {{count}} keys missing them (files: countMatches/countModified/countFiles, git: openCount, home: windowHours/windowMinutes/statRunsInLast7Days/statAcrossRuns/statActiveDaysInLast7, prompts: metaHasPrompts, workspaces: moreErrors) in both en and zh-CN, and removed the 5 legacy v3 _plural keys. A full-suite scan confirms zero remaining v3 _plural keys and zero {{count}} base keys without _other in either locale. English plural forms are distinct where the language requires (e.g. "{{count}} matches"); zh-CN uses identical _one/_other values as Chinese does not inflect.

2. Live docs — replaced the three historical phase notes (i18n-phase1/2/3-plan.md) with a single live-state guide at docs/localization.md covering the current architecture (21 namespaces, en/zh-CN), v4 plural rules, the extract:i18n workflow including the optionKeyLabel/metaKeyLabel false-positive cleanup step, file format conventions (BOM warning), the module-scope i18n.t() ban, and current limitations. No dangling references (verified: only the phase docs referenced each other).

Verification: 1032/1032 tests pass, i18n parity 31/31.

All three addressed in 1d24232:

1. Native menu/tray (src-tauri/src/lib.rs + i18n.rs): the menu and the macOS tray now resolve the persisted language through the new i18n::resolve_ui_language, which uses the saved preference when present and otherwise falls back to the host locale via the sys-locale crate (zh* -> zh-CN, else en). This matches the frontend detectInitialLanguage semantics, so on first launch on a zh-CN host the native menu/tray are Chinese instead of English. A saved preference still wins afterwards.

2. Composer follow-up hint (Composer.tsx + composer.json): behavior now interpolates the translated composer:steer/composer:queue labels, and action uses two new localized phrases composer:followUpActionSteer ("steer" / "引导") and followUpActionQueue ("queue" / "排队"). The zh-CN followUpDefault template drops the awkward "以" so it reads naturally with Chinese verbs. No mixed-language hint anymore.

3. Rust test assertion (src-tauri/src/types.rs): app_settings_defaults_from_empty_json now expects "" for the no-preference default, consistent with the new semantics.

Verified: 1032/1032 tests pass, typecheck clean. Rust changes parse cleanly via cargo fmt; cargo check remains blocked by the pre-existing upstream whisper-rs build failure (#599). sys-locale 0.3 is a pure-Rust, dependency-free crate (Windows uses GetUserDefaultUILanguage, macOS CoreFoundation, Unix env vars), consistent with the host-locale detection approach.

All four addressed in 31db2d4:

1. P1 i18n.t binding (4 files): const { t } = i18n module-level destructuring in usageLabels.ts, homeFormatters.ts, homeUsageViewModel.ts and ComposerAttachments.tsx dropped the i18next instance's this, so the first t(...) call at render threw. All four now use i18n.t.bind(i18n); since they're plain functions evaluated per call (no memoized results), language switching still applies immediately. (Test suite didn't catch it because these modules aren't exercised through their real render paths.) While in this file I also localized the home usage card's hardcoded Updated prefix via the new home:updatedLabel key.

2. Tray rebuild paths (src-tauri/src/tray.rs): both macOS rebuild entry points — recent-threads update (set_tray_recent_threads) and session usage update (set_tray_session_usage) — now run the saved language through i18n::resolve_ui_language before update_tray_menu, so an empty saved value no longer snaps the tray back to English after the initial Chinese tray was built at startup. The about-window title path in menu.rs::handle_menu_event got the same treatment (including the lock-failure fallback).

3. Trans placeholder (SettingsGitSection.tsx): settings:git_commit_message_prompt_help contains {{diff}}, so <Trans> treated it as a missing interpolation and dropped it. It now passes values={{ diff: "{diff}" }}, rendering the literal {diff} placeholder again in both locales.

4. docs/i18n-glossary.md: decision #5 rewritten to describe the current state — Rust menu/tray/about-title are localized via t_rs/t_rs_fmt with host-locale fallback through resolve_ui_language — instead of the stale "not in Phase 1 scope" note.

Verified: 1032/1032 tests pass, typecheck clean, cargo fmt --check clean on the touched Rust files. cargo check remains blocked by the pre-existing upstream whisper-rs failure (#599).

All three addressed in 4c57a49:

1. Cargo.lock: committed with the sys-locale v0.3.2 entry (its only dependency, libc, is already in the lock), so locked/reproducible Rust builds are consistent again. Note: sys-locale 0.3.2 itself compiles cleanly — the remaining cargo check errors are all the pre-existing upstream whisper-rs breakage (#599).

2. zh-CN server_remote_use: now keeps the {{name}} interpolation — "使用 {{name}}" — matching the en resource, so inactive remote buttons and their aria-labels retain the remote-specific context instead of all reading the bare "使用" label.

3. GitDiffPanel: the pushSyncConflictMessage memo now includes t in its dependency array, so the localized conflict text refreshes on language change even when the push error payload is unchanged.

Verified: 1032/1032 tests pass, typecheck clean.