refactor: organize frontend components into features/ directories

Moved 43 component files into 9 feature domains:
- features/git/ — commit-dialog, commit-panel, file-editor, git-mount-editor,
  git-toolbar, merge-dialog
- features/project/ — repositories-settings-tab, repository-create-dialog
- features/terminal/ — special-keys-panel, special-keys-strip,
  terminal-session-tabs, terminal
- features/workspace/ — workspace-card, workspace-create-form,
  workspace-header, workspace-instance-chips
- features/session/ — create-session-form, session-card, session-list
- features/tool/ — instance-list, manifest-editor, start-tool-fab,
  start-tool-modal, tool-starter, tools-bottom-sheet
- features/notification/ — event-toast-bridge, notification-center,
  notification-item
- features/settings/ — settings-tab-layout
- features/mobile/ — mobile-action-sheet, mobile-detail-view, mobile-edit-view,
  mobile-fab, mobile-list-view, mobile-nav, mobile-page-header,
  mobile-terminal-header, mobile-terminal-wrapper

Updated all imports across pages and components.
Root components/ now only contains generic UI pieces:
app-shell, code-editor, data-states, icon, protected-route, syntax-highlighter.

Quality gates: verified no remaining old imports.

apps/web/src/components/features/mobile/mobile-fab.tsx

@@ -0,0 +1,22 @@
+import { Icon } from "./icon";
+
+interface MobileFABProps {
+  onClick: () => void;
+  label?: string;
+}
+
+export const MobileFAB: React.FC<MobileFABProps> = ({
+  onClick,
+  label = "Create new",
+}) => {
+  return (
+    <button
+      className="mobile-fab"
+      onClick={onClick}
+      type="button"
+      aria-label={label}
+    >
+      <Icon name="add" size="md" />
+    </button>
+  );
+};