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/components/HoverEditButton.tsx

@@ -4,26 +4,48 @@ import { Button } from "@/components/ui/button";
 interface HoverEditButtonProps {
 	onClick: () => void;
 	label?: string;
+	/** Controls visibility below the `md:` (768px) breakpoint.
+	 *
+	 * - `always` (default): the button is always visible on mobile/touch.
+	 * - `hover`: keep the legacy opacity-0-everywhere behavior.
+	 *
+	 * At `md:` and above the hover-reveal aesthetic is always preserved
+	 * (`md:opacity-0 md:group-hover:opacity-100`), so desktop is not regressed.
+	 * See OpenSpec change `mobile-responsive-parity`, spec R5. */
+	mobile?: "always" | "hover";
 }
 
 /**
- * Hover-to-reveal edit affordance.
+ * Hover-to-reveal edit affordance (desktop) / always-visible (mobile).
  *
- * Keeps the `rail-edit` class plus the opacity-0 base + transition so the
+ * Keeps the `rail-edit` class plus the opacity base + transition so the
  * existing hover-reveal rules in consuming pages (Actions, Settings) still
  * target it (`&:hover .rail-edit { opacity: 1 }`) until those pages migrate.
- * MUI IconButton + EditOutlined → shadcn `Button variant="ghost" size="icon-sm"`
- * + lucide `Pencil`. Same exported props/display name.
+ *
+ * Mobile behavior (`mobile="always"`, the default): the button is visible by
+ * default below `md` because hover does not fire on touch. The hover-reveal
+ * aesthetic is layered back on at `md:` and above via `md:opacity-0
+ * md:group-hover:opacity-100`. MUI IconButton + EditOutlined → shadcn `Button
+ * variant="ghost" size="icon-sm"` + lucide `Pencil`. Same exported props/display
+ * name. See OpenSpec change `mobile-responsive-parity`, spec R5.
  */
 export function HoverEditButton({
 	onClick,
 	label = "Edit",
+	mobile = "always",
 }: HoverEditButtonProps) {
+	// Legacy mode: opacity-0 everywhere, revealed by group hover (the consuming
+	// row supplies `group`).
+	const hoverClasses =
+		mobile === "hover"
+			? "opacity-0 transition-opacity duration-100 ease-out group-hover:opacity-100"
+			: "md:opacity-0 md:transition-opacity md:duration-100 md:ease-out md:group-hover:opacity-100";
+
 	return (
 		<Button
 			variant="ghost"
 			size="icon-sm"
-			className="rail-edit text-muted-foreground opacity-0 transition-opacity duration-100 ease-out"
+			className={`rail-edit text-muted-foreground mobile-touch-target ${hoverClasses}`}
 			aria-label={label}
 			onMouseDown={(e) => e.stopPropagation()}
 			onClick={(e) => {