feat: floating action button for starting tools globally

- New StartToolFAB component: fixed floating button (bottom-right) opens
  a modal with workspace selector + ToolStarter
- Added to AppShell: available on every page except mobile terminal
- Dashboard (home): removed old CreateSessionForm and 'Quick create' section,
  replaced with FAB + 'Workspaces quick access' prompt
- Sessions page: removed inline workspace selector + ToolStarter, now
  shows prompt to use the FAB
- Styles: .start-tool-fab with hover scale, shadow, mobile offset above tab bar

Quality gates: tsc --noEmit clean, pytest workspaces API (9 passed, 1 skipped)

apps/web/src/components/app-shell.tsx

@@ -14,6 +14,7 @@ import { EventToastBridge } from "./event-toast-bridge";
 import { NotificationCenter } from "./notification-center";
 import { Icon } from "./icon";
 import { MobileNav } from "./mobile-nav";
+import { StartToolFAB } from "./start-tool-fab";
 import type { IconName } from "../utils/icons";
 
 const NAV_ITEMS: {
@@ -170,6 +171,7 @@ export const AppShell = () => {
 								}
 							/>
 						)}
+						<StartToolFAB />
 					</div>
 				</NotificationProvider>
 			</ToastProvider>