Docs + verify report for mobile responsive parity (Slice 10)

Add Mobile Responsive Design section to docs/REQUIREMENTS.md documenting
the breakpoint policy (single md:768px), hybrid table strategy (cards below
md), SheetForm edit flows, 44px touch targets, dashboard single-column +
anchors, unchanged polling, and HoverEditButton behavior.

Add openspec verify-report.md with per-AC evidence (AC1-AC8), residual
risks (R4.5 dirty-state confirm, default-button touch targets, polling on
battery, iOS Safari manual verification, pagination duplication), and
non-goals confirmation.

All 9 routes fully operable at 375px. 116 frontend tests pass; lint/build
green. Desktop layout unchanged. No backend changes.

Refs openspec/changes/mobile-responsive-parity/ (tasks slice 10).

docs/REQUIREMENTS.md

@@ -468,3 +468,57 @@ The system receives backup execution reports from an external backup tool via HT
 
 - Backup tool uses auto-generated Bearer API key
 - Frontend uses existing OIDC/JWT auth
+
+## Mobile Responsive Design
+
+The frontend is fully operable in phone portrait (≥360px) at a single `md:`
+(768px) breakpoint. Tablets and wider viewports use the desktop layout
+unchanged.
+
+### Breakpoint policy
+
+- Single responsive cut: `md:` (768px). Below is "mobile"; at-or-above is
+  "desktop" (existing layout, unchanged).
+- `useIsMobile()` hook (`frontend/src/hooks/useIsMobile.ts`) is the single
+  source of truth; it wraps `matchMedia("(max-width: 768px)")` and is SSR-safe.
+- No `sm:` intermediate cut. No PWA, manifest, or service worker.
+
+### Data tables (hybrid)
+
+- The four wide tables (Media, FileBrowser, Users, Backups) render stacked
+  **cards per row** below `md` via `MobileCardRow`, each showing a primary
+  title plus 3–5 key fields. Narrow tables (SessionActivity) keep horizontal
+  scroll. The TanStack column-visibility toggle is hidden below `md`.
+- At `md:` and above, all tables render as the existing `<DataTable>` unchanged.
+
+### Edit forms (Sheet)
+
+- Below `md`, ServicePage, Settings (machine editor), message compose, and
+  WidgetConfigDialog open inside a full-height `SheetForm` (side=bottom,
+  `h-[100dvh]`) with sticky header + sticky save bar instead of a centered
+  Dialog.
+- At `md:` and above, the existing Dialog-based forms are unchanged.
+
+### Touch targets
+
+- All interactive elements below `md` have a minimum 44×44px hit area via the
+  `.mobile-touch-target` CSS utility (applied only below 768px). This covers
+  icon buttons, checkboxes, switches, and small text buttons. The class is a
+  no-op at `md:` and above.
+
+### Dashboard
+
+- Below `md`, the widget grid collapses to a single column with a section
+  anchor bar (Observability / Media / Backups / Custom) for quick navigation.
+- At `md:` and above, the existing multi-widget grid is unchanged.
+
+### Polling
+
+- Widget refresh intervals and the message-queue poll interval are identical
+  on mobile and desktop. A follow-up to pause refetch when the tab is hidden
+  (`document.visibilityState`) is tracked as a future battery optimization.
+
+### `HoverEditButton`
+
+- Below `md`, edit affordances are always visible (not hover-gated). At `md:`
+  and above, the desktop hover-reveal aesthetic is preserved.