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/hooks/useIsMobile.ts

@@ -0,0 +1,32 @@
+import { useEffect, useState } from "react";
+
+/** Mobile breakpoint (must match Tailwind `md:` and the OpenSpec spec R1.2). */
+const MOBILE_QUERY = "(max-width: 768px)";
+
+/**
+ * Single source of truth for the mobile/desktop responsive cut.
+ *
+ * Returns `true` when the viewport matches `max-width: 768px` (phone portrait),
+ * `false` at `md:` and above. SSR-safe: returns `false` when `window` is
+ * undefined so server-rendered markup stays on the desktop path.
+ *
+ * Replaces the ad-hoc `window.matchMedia("(max-width: 768px)")` reads scattered
+ * across pages (App.tsx, Media.tsx) — see OpenSpec change
+ * `mobile-responsive-parity`, design §`useIsMobile`.
+ */
+export function useIsMobile(): boolean {
+	const [isMobile, setIsMobile] = useState(
+		() =>
+			typeof window !== "undefined" && window.matchMedia(MOBILE_QUERY).matches,
+	);
+
+	useEffect(() => {
+		if (typeof window === "undefined") return;
+		const mql = window.matchMedia(MOBILE_QUERY);
+		const handler = (e: MediaQueryListEvent) => setIsMobile(e.matches);
+		mql.addEventListener("change", handler);
+		return () => mql.removeEventListener("change", handler);
+	}, []);
+
+	return isMobile;
+}