feat(03-07): InstallPrompt — iOS walkthrough banner + Android beforeinstallprompt

- Implement isIOSSafariNonStandalone(): iOS UA + navigator.standalone detection
- Implement useAndroidInstallPrompt(): captures beforeinstallprompt, exposes canInstall/triggerInstall
- InstallPrompt renders nothing when display-mode:standalone or navigator.standalone (already installed)
- iOS branch: dismissible banner with 'Install FamilySync' heading, 'How to install' link
  opens 5-step walkthrough sheet (exact UI-SPEC copy, orange #F5A623 step number annotation)
- Android branch: banner with 'Install' button shown only when canInstall=true
- localStorage.installPromptDismissed persists banner dismissal
- role="banner", dismiss aria-label="Dismiss install prompt", 44px touch targets
- Mount <InstallPrompt /> in CalendarShell (phone: below AppNav; desktop: top of content area)
- InstallPrompt.test.tsx GREEN (5 behavior tests); full PWA suite 44 tests green; tsc clean

apps/pwa/src/components/CalendarShell.tsx

@@ -48,6 +48,7 @@ import { EventDetailPopover } from './EventDetailPopover.js'
 import { AppNav } from './AppNav.js'
 import { ColorLegend } from './ColorLegend.js'
 import { SkeletonCalendar } from './SkeletonCalendar.js'
+import { InstallPrompt } from './InstallPrompt.js'
 
 // ── Helpers ────────────────────────────────────────────────────────────────
 
@@ -314,6 +315,7 @@ export function CalendarShell() {
           currentUserColor={meQuery.data?.user.color}
           currentUserName={meQuery.data?.user.displayName ?? undefined}
         />
+        <InstallPrompt />
         <CalendarContent />
         <EventDetailPopover />
       </div>
@@ -341,7 +343,10 @@ export function CalendarShell() {
       />
 
       {/* Main content area */}
-      <CalendarContent />
+      <div style={{ flex: 1, minWidth: 0, display: 'flex', flexDirection: 'column', overflow: 'hidden' }}>
+        <InstallPrompt />
+        <CalendarContent />
+      </div>
 
       {/* EventDetailPopover — standalone mode driven by Zustand openEventId */}
       <EventDetailPopover />