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/styles.css

@@ -5615,3 +5615,57 @@ a:active,
 .tool-starter-header h4 {
 	margin: 0;
 }
+
+/* ─── Floating Action Button ─── */
+.start-tool-fab {
+	position: fixed;
+	bottom: 2rem;
+	right: 2rem;
+	z-index: 100;
+	width: 3.5rem;
+	height: 3.5rem;
+	border-radius: 50%;
+	background: var(--primary);
+	color: var(--primary-fg);
+	border: none;
+	box-shadow: 0 4px 14px rgba(0, 0, 0, 0.25);
+	cursor: pointer;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	transition: transform 0.15s ease, box-shadow 0.15s ease;
+}
+
+.start-tool-fab:hover {
+	transform: scale(1.08);
+	box-shadow: 0 6px 20px rgba(0, 0, 0, 0.3);
+}
+
+.start-tool-fab:active {
+	transform: scale(0.95);
+}
+
+.start-tool-modal {
+	max-width: 480px;
+	width: 90vw;
+}
+
+.modal-header {
+	display: flex;
+	justify-content: space-between;
+	align-items: center;
+	margin-bottom: var(--space-3);
+}
+
+.modal-header h3 {
+	margin: 0;
+}
+
+@media (max-width: 640px) {
+	.start-tool-fab {
+		bottom: 5rem; /* above mobile tab bar */
+		right: 1rem;
+		width: 3rem;
+		height: 3rem;
+	}
+}