feat: refactor composer to two-layer layout, add session permission & context indicator

Composer restructure:
- MessageInput internal footer now only contains: file button, model
  selector, slash command button, and send button
- Removed mode toggle (Code/Plan) from composer area (logic preserved)
- New ChatComposerActionBar below input with three slots: left (Design
  Agent), center (permission selector), right (context indicator)

New components:
- SlashCommandButton: inserts `/` at cursor, reuses existing slash
  command detection and popover logic
- ChatPermissionSelector: session-level permission toggle (default /
  full_access) with danger confirmation dialog
- ContextUsageIndicator: SVG donut chart showing context usage ratio
  with HoverCard displaying model, tokens, cache stats
- ChatComposerActionBar: slot-based container for action bar items

Session permission (permission_profile):
- DB: new column on chat_sessions, default 'default'
- API: PATCH sessions supports updating permission_profile
- Chat API: bypassPermissions = global setting OR session full_access
- PermissionPrompt: auto-approves and hides when full_access
- Claude client: combines global and per-session bypass flags

Bridge integration:
- permission-broker: auto-approves permissions for full_access sessions
  without sending IM notifications
- conversation-engine: passes bypassPermissions to SDK for full_access
- PATCH route: auto-approves pending bridge permissions on switch to
  full_access

Other:
- ImageGenToggle relabeled to "Design Agent" / "设计 Agent"
- New model-context.ts with MODEL_CONTEXT_WINDOWS lookup
- Provider models API returns optional contextWindow field
- New useContextUsage hook for token usage calculation
- i18n: added composer.*, permission.*, context.* keys (en + zh)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/chat/ImageGenToggle.tsx

@@ -31,11 +31,11 @@ export function ImageGenToggle() {
               : 'text-muted-foreground border-border/60 hover:text-foreground hover:border-foreground/30 hover:bg-accent/50'
           )}
         >
-          {t('imageGen.toggleLabel' as TranslationKey)}
+          {t('composer.designAgent' as TranslationKey)}
         </button>
       </TooltipTrigger>
       <TooltipContent>
-        {t('imageGen.toggleTooltip' as TranslationKey)}
+        {t('composer.designAgentTooltip' as TranslationKey)}
       </TooltipContent>
     </Tooltip>
   );