feat: add keyboard shortcuts with platform-aware modifier keys

[samzong]

Summary

• Add global keyboard shortcuts: Cmd/Ctrl+Shift+O (new task), Cmd/Ctrl+Shift+F (file browser), Cmd/Ctrl+K (focus search)
• Add send shortcut toggle (Enter vs Cmd/Ctrl+Enter) in Settings, persisted to AppConfig
• Fix macOS shortcut detection: switch from e.key to e.code for reliable modifier+key combos
• Fix platform-aware modifier symbol: show ⌘ on macOS, Ctrl on Windows/Linux (Settings UI + ChatInput hint)

Bug Fixes

• macOS shortcuts not firing: e.key produces unreliable values when Cmd+Shift is held; e.code uses physical key identity
• Redundant disk write on startup: hydrating sendShortcut from config no longer triggers updateSettings() back to disk
• Settings no-op click: toggling to the already-selected shortcut option no longer fires toast/write

Changed Files

File	Change
App.tsx	Global shortcut listener (e.code), hydrate sendShortcut via setState()
ChatInput.tsx	Send shortcut toggle logic, platform-aware hint text
Settings/index.tsx	Send shortcut toggle UI with guard, modKey interpolation
uiStore.ts	SendShortcut type, setSendShortcut, searchFocusTrigger/focusSearch
LeftNav/index.tsx	searchInputRef + searchFocusTrigger effect
utils.ts	isMac, modKey constants
config.ts, clawwork.d.ts	sendShortcut field in AppConfig/AppSettings
en.json, zh.json	i18n keys with {{mod}} interpolation

Verification

tsc --noEmit passes with zero errors.

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