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/components/ProjectSidebar.tsx

@@ -1,8 +1,9 @@
 import { useEffect, useMemo, useRef, useState } from 'react';
 import { NavLink, useLocation, useNavigate } from 'react-router-dom';
-import { ChevronRight, ExternalLink, Folder, MessageSquare, Plus } from 'lucide-react';
+import { ChevronRight, ExternalLink, Folder, MessageSquare, Plus, Settings as SettingsIcon } from 'lucide-react';
 import { toast } from 'sonner';
 import { Button } from '@/components/ui/button';
+import { sessionEvents } from '@/hooks/sessionEvents';
 import {
   ContextMenu,
   ContextMenuContent,
@@ -198,7 +199,7 @@ export function ProjectSidebar() {
   const rowCls = (active: boolean) =>
     active ? 'bg-sidebar-accent text-sidebar-accent-foreground' : 'hover:bg-sidebar-accent/60';
 
-  const { open: drawerOpen } = useSidebarDrawer();
+  const { open: drawerOpen, setOpen: setDrawerOpen } = useSidebarDrawer();
   const { isMobile } = useViewport();
   const pull = usePullToRefresh(() => retry(), { enabled: isMobile });
 
@@ -412,6 +413,30 @@ export function ProjectSidebar() {
           })}
       </nav>
 
+      {/* v1.9: bottom-pinned Settings button. In a session, opens/focuses the
+          workspace settings pane via the sessionEvents bus (Session.tsx owns
+          the panesHook). Outside a session there's no workspace to mount the
+          pane in, so we navigate to /settings (themes page) instead. */}
+      <div className="border-t shrink-0 p-2">
+        <button
+          type="button"
+          onClick={() => {
+            if (activeSession) {
+              sessionEvents.emit({ type: 'open_settings_pane' });
+              if (isMobile) setDrawerOpen(false);
+            } else {
+              navigate('/settings');
+              if (isMobile) setDrawerOpen(false);
+            }
+          }}
+          className="w-full flex items-center gap-2 px-2 py-1.5 rounded-md text-sm hover:bg-sidebar-accent/60 text-sidebar-foreground"
+          aria-label="Settings"
+        >
+          <SettingsIcon className="size-3.5 shrink-0 opacity-70" />
+          <span className="flex-1 text-left">Settings</span>
+        </button>
+      </div>
+
       <AddProjectModal open={addOpen} onOpenChange={setAddOpen} onAdded={() => {}} />
 
       <Dialog open={archiveProjectConfirm !== null} onOpenChange={(open) => { if (!open) setArchiveProjectConfirm(null); }}>