v2.0.0: BooCoder frontend — chat pane + diff pane + session picker

Integrates BooCoder as a 'coder' workspace pane within the existing
BooChat SPA at code.indifferentketchup.com. Renamed the placeholder
'agent' pane kind to 'coder' across all types, menus, hooks, and
mobile switcher (Icon: Code instead of Bot).

CoderPane.tsx: split layout with chat area (messages via WS to
boocoder:9502, input bar posting to /api/coder/sessions/:id/messages)
and diff panel (pending changes with Approve/Reject per change plus
Approve All/Reject All). Reuses MarkdownRenderer for message content.

Proxy: Vite dev config adds /api/coder → boocoder:9502 (ordered above
/api per CLAUDE.md proxy-ordering rule). Production: Fastify route in
apps/server/src/index.ts proxies /api/coder/* to http://boocoder:3000
via fetch() pass-through. WS connects directly to :9502 (same
Tailscale network, no proxy needed for WebSocket upgrade).

WorkspacePaneKind mirror updated in both apps/web and apps/server
types. useWorkspacePanes gains coderPane() factory (replaces the old
agent toast stub). Workspace.tsx switch renders CoderPane for
pane.kind === 'coder'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/src/pages/Session.tsx

@@ -178,7 +178,7 @@ function SessionInner({ sessionId }: { sessionId: string }) {
   // the new pane's id to the URL atomically so the effect's next pass sees a
   // matching id and is a no-op. Desktop has no URL pane state — fall through.
   const addPaneAndSwitch = useCallback(
-    (kind: 'chat' | 'terminal' | 'agent') => {
+    (kind: 'chat' | 'terminal' | 'coder') => {
       const newPaneId = addSplitPane(kind);
       if (newPaneId === null) return;
       if (isMobile) {