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/pages/Session.tsx

@@ -6,7 +6,7 @@ import {
   useParams,
   useSearchParams,
 } from 'react-router-dom';
-import { ChevronRight, FolderTree, Menu } from 'lucide-react';
+import { ChevronRight, FolderTree, Menu, X } from 'lucide-react';
 import { api } from '@/api/client';
 import type { Project, Session as SessionType } from '@/api/types';
 import { sessionEvents } from '@/hooks/sessionEvents';
@@ -61,6 +61,9 @@ function SessionInner({ sessionId }: { sessionId: string }) {
     initializeFirstChatIfEmpty,
     validatePanes,
   } = panesHook;
+  const [coderConnected, setCoderConnected] = useState<Record<string, boolean>>({});
+  const activePane = panes[activePaneIdx];
+  const activeIsCoder = activePane?.kind === 'coder';
 
   const openChatInActivePane = useCallback(
     (chatId: string) => openChatInPane(activePaneIdxRef.current, chatId),
@@ -402,6 +405,16 @@ function SessionInner({ sessionId }: { sessionId: string }) {
                 onAddPane={addPaneAndSwitch}
                 disabled={panes.length >= MAX_PANES}
               />
+              {activeIsCoder && activePane && panes.length > 1 && (
+                <button
+                  type="button"
+                  onClick={() => removePane(activePaneIdx)}
+                  className="inline-flex items-center justify-center min-h-[44px] min-w-[44px] rounded-full bg-muted/40 hover:bg-muted/70 text-foreground shrink-0"
+                  aria-label="Close pane"
+                >
+                  <X size={16} />
+                </button>
+              )}
             </div>
           </>
         ) : (
@@ -495,6 +508,11 @@ function SessionInner({ sessionId }: { sessionId: string }) {
           session={session}
           project={project}
           onAddPane={addPaneAndSwitch}
+          onCoderConnectedChange={(paneId, connected) =>
+            setCoderConnected((prev) =>
+              prev[paneId] === connected ? prev : { ...prev, [paneId]: connected },
+            )
+          }
         />
       )}
     </div>