Add mobile responsive primitives (Slice 1)

Foundation for the mobile-responsive-parity change. Adds:
- useIsMobile() hook: single source of truth for the md:768px cut (SSR-safe)
- MobileCardRow<T>: stacked card list for wide tables below md, with getRowId
  stable keys, primary field as title, optional onRowClick + actions slot
- SheetForm: full-height form host (h-[100dvh], flex column, sticky header +
  footer via flex not position:sticky) for mobile edit flows
- HoverEditButton: mobile prop (default 'always') -- always visible below md,
  hover-revealed at md+; desktop aesthetic preserved
- .mobile-touch-target CSS utility: 44x44 min hit area below md (WCAG 2.5.5)
- App.tsx refactored to use useIsMobile(); shell behavior unchanged

Tests cover primary/field rendering, onRowClick, actions slot, empty rows,
no-primary, stable keys (no duplicate-key warning), and all SheetForm
interactions. 86 tests pass; lint/build green.

MobileCardRow key strategy: uses getRowId when provided (falls back to index);
per design §trade-offs, fields are declared per-table to prioritize by mobile
importance rather than auto-derived from column defs.

Refs openspec/changes/mobile-responsive-parity/ (design §Shared primitives,
spec R1/R5/R6, tasks slice 1).

frontend/src/App.tsx

@@ -28,6 +28,7 @@ import { getOidcConfig, isOidcConfigured, setAccessToken } from "./auth";
 import { fetchAppVersion } from "./api/client";
 import { FRONTEND_VERSION_LABEL } from "./version";
 import { usePersistentState } from "./hooks/usePersistentState";
+import { useIsMobile } from "./hooks/useIsMobile";
 import { Button } from "@/components/ui/button";
 import {
 	Tooltip,
@@ -316,16 +317,7 @@ function ShellLayout({
 	onToggleDarkMode: () => void;
 }) {
 	const [sidebarCollapsed, setSidebarCollapsed] = useState(false);
-	const [isMobile, setIsMobile] = useState(
-		() => window.matchMedia("(max-width: 768px)").matches,
-	);
-
-	useEffect(() => {
-		const mql = window.matchMedia("(max-width: 768px)");
-		const handler = (e: MediaQueryListEvent) => setIsMobile(e.matches);
-		mql.addEventListener("change", handler);
-		return () => mql.removeEventListener("change", handler);
-	}, []);
+	const isMobile = useIsMobile();
 
 	return (
 		<div className="min-h-screen bg-background">