web(coder UI): ChatInput migration + Thinking render + DiffPanel route fix

Bundles in-progress working-tree UI work not authored this session (CoderPane ChatInput migration, AgentComposerBar/CoderMessageList/tab-bar/sidebar/pane refinements, provider icons) with this session's changes to the same files: MessageBubble renders a collapsible 'Thinking' block from reasoning_text/reasoning_parts (surfacing ACP agent_thought_chunk + native reasoning), and the DiffPanel approve/reject calls are repointed to the real /api/coder/pending/:id/apply and /reject routes (the old /sessions/:id/pending/:id/approve|reject paths did not exist).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/web/src/components/ChatTabBar.tsx

@@ -181,7 +181,7 @@ export function ChatTabBar({
               <Plus size={12} />
             </button>
           </DropdownMenuTrigger>
-          <DropdownMenuContent align="end" className="min-w-40">
+          <DropdownMenuContent align="end" className="w-fit">
             <DropdownMenuItem onSelect={() => onAddPane('chat')}>
               <MessageSquare size={14} /> New BooChat
             </DropdownMenuItem>