v1.9: settings pane + per-project defaults + bulk archive + themes lift

Adds a singleton, ephemeral 'settings' pane kind to the workspace.
Opened via a new bottom-pinned button in ProjectSidebar (emits an
open_settings_pane event when a session is mounted; navigates to
/settings otherwise). Pane has three sections — Session, Project,
Theme — and a maximize toggle that hides sibling pane columns via
display:none on desktop only. Settings panes don't count toward
MAX_PANES and are filtered out of the localStorage persistence layer
so reload always restores a clean workspace.

Schema (additive):
- projects.default_system_prompt TEXT NOT NULL DEFAULT ''
- projects.default_web_search_enabled BOOLEAN NOT NULL DEFAULT false
- sessions.web_search_enabled BOOLEAN  (nullable; null = inherit)

Inference resolves user_prompt = session.system_prompt.trim() ||
project.default_system_prompt.trim() — empty/whitespace at either
layer means "no override". Keeps the columns NOT NULL and matches
the existing inherit semantics.

Server routes:
- GET /api/projects/:id (new; settings pane refetches on
  project_updated)
- PATCH /api/projects/:id accepts default_system_prompt,
  default_web_search_enabled
- PATCH /api/sessions/:id accepts web_search_enabled (tri-state)
- POST /api/projects/:id/sessions/archive-all + GET
  /api/projects/:id/sessions/open-count
- POST /api/sessions/:id/chats/archive-all + GET
  /api/sessions/:id/chats/open-count
- PATCH /api/sessions/:id now broadcasts session_updated on every
  successful PATCH (was rename-only). Lets SettingsPane open in
  another tab pick up edits without a refetch.

Bulk-archive publishes one session_archived / chat_archived frame
per affected id so useSidebar's existing reducer cases handle them
incrementally — no new frame type, no payload widening.

ModelPicker refactored: shared ModelList inside a responsive shell.
Desktop = labeled trigger + DropdownMenu, mobile = icon-only Cpu
button + BottomSheet. Header in Session.tsx drops the pill wrap on
mobile since the new trigger is the visual.

ChatInput gains an icon-only '+' DropdownMenu next to AgentPicker
when sessionId + webSearchEnabled props are provided. One item for
now — Web search — with a checkmark reflecting the stored value
(true), not the effective one. Click PATCHes the override; to
restore inherit-from-project the user opens SettingsPane.

ThemePicker lifted out of pages/Settings.tsx into a reusable
component. The standalone /settings route is now a thin wrapper
that mounts <ThemePicker /> with a Back button on top
(navigate(-1) with fallback to '/'); the SettingsPane Theme tab
renders the same picker bare.

Project section delete-flow removed (button + confirm dialog +
handler). Replaced with "Archive all sessions" using the same
two-step count → confirm → fire pattern as "Archive all chats" in
the Session section. api.projects.remove() stays in the client
because useProjects.ts still uses it.

Hand-rolled Switch primitive in SettingsPane (no shadcn switch in
the project; spec said no new deps). Section nav is plain buttons
(no shadcn Tabs).

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

apps/web/src/pages/Session.tsx

@@ -116,9 +116,32 @@ function SessionInner({ sessionId }: { sessionId: string }) {
         event.session_id === sessionId
       ) {
         navigate(`/project/${event.project_id}`);
+        return;
+      }
+      // v1.9: any session_updated for this session triggers a full refetch so
+      // SettingsPane (mounted in a workspace pane) picks up system_prompt /
+      // web_search_enabled / model edits made from another tab.
+      if (event.type === 'session_updated' && event.session_id === sessionId) {
+        void api.sessions.get(sessionId).then((s) => {
+          setSession(s);
+          setName((prev) => (editingName ? prev : s.name));
+        }).catch(() => {});
+        return;
+      }
+      // v1.9: project_updated → refetch project so the Project section in
+      // SettingsPane reflects the new defaults.
+      if (event.type === 'project_updated' && project && event.project_id === project.id) {
+        void api.projects.get(project.id).then(setProject).catch(() => {});
+        return;
+      }
+      // v1.9: sidebar Settings button broadcasts this when a session is
+      // mounted; we own the workspace pane state, so we open/focus the
+      // singleton settings pane here.
+      if (event.type === 'open_settings_pane') {
+        panesHook.openOrFocusSettingsPane();
       }
     });
-  }, [sessionId, editingName, navigate]);
+  }, [sessionId, editingName, navigate, project, panesHook]);
 
   // v1.8: URL ?pane= sync (mobile only). Lifted from Workspace.tsx so
   // MobileTabSwitcher's onSwitchPane can push the same URL state and the
@@ -211,15 +234,13 @@ function SessionInner({ sessionId }: { sessionId: string }) {
               </div>
 
               {session && (
-                <div className="inline-flex items-center rounded-full bg-muted/40 hover:bg-muted/70 px-1 shrink-0">
-                  <ModelPicker
-                    value={session.model}
-                    onChange={async (model) => {
-                      const updated = await api.sessions.update(session.id, { model });
-                      setSession(updated);
-                    }}
-                  />
-                </div>
+                <ModelPicker
+                  value={session.model}
+                  onChange={async (model) => {
+                    const updated = await api.sessions.update(session.id, { model });
+                    setSession(updated);
+                  }}
+                />
               )}
 
               <button
@@ -337,6 +358,8 @@ function SessionInner({ sessionId }: { sessionId: string }) {
           }}
           panesHook={panesHook}
           chatsHook={chatsHook}
+          session={session}
+          project={project}
         />
       )}
     </div>