feat(desktop): per-session profile switching + cross-profile sessions

Add first-class profile support to the desktop app without app reloads.

- Swap the single live gateway onto a session's profile lazily (spawned on
  demand by the Electron backend pool), so one backend serves the active
  profile and others stay cold — no OOM with many profiles.
- Aggregate sessions across profiles by reading each profile's state.db
  read-only; unified "All profiles" view groups sessions per profile with
  per-profile pagination, while the default view stays scoped to one profile.
- Add an Arc-style profile rail at the sidebar foot: a default<->all toggle
  pinned left, colored named-profile squares scrolling between, Manage pinned
  right. Profile identity is a deterministic per-name color.
- Route profile-scoped REST (config/env/skills/tools/model) to the active
  gateway profile and invalidate React Query caches on swap. Single-profile
  users never trigger a swap, so their path is unchanged.

Backend:
- web_server: profile-aware active/list endpoints + per-profile session
  totals; hermes_state: session_count(exclude_children); main.py: honor
  --profile over HERMES_HOME env for pooled backends.

UI primitives:
- Add a position-aware Tip tooltip (instant, themed) as a drop-in for native
  title=, and strip redundant tooltips from self-descriptive chrome.

apps/desktop/src/app/shell/app-shell.tsx

@@ -2,6 +2,7 @@ import { useStore } from '@nanostores/react'
 import type { CSSProperties, ReactNode } from 'react'
 import { useSyncExternalStore } from 'react'
 
+import { NotificationStack } from '@/components/notifications'
 import { PaneShell } from '@/components/pane-shell'
 import { SidebarProvider } from '@/components/ui/sidebar'
 import {
@@ -153,6 +154,10 @@ export function AppShell({
       </main>
 
       {overlays}
+
+      {/* Mounted at the shell root (after overlays) so success/error toasts
+          surface above every route and overlay — not just the chat view. */}
+      <NotificationStack />
     </SidebarProvider>
   )
 }