feat: add PWA install prompt UI (#35)

- Create usePWAInstall composable for install management
- Add InstallPrompt banner component with auto-show after 3s
- Add App Settings tab in settings page
- Show install button with loading state
- Display installation status and instructions
- Handle dismissal with 7-day cooldown
- Add iOS/Android installation guides
- Show PWA features list
- Display storage usage with visual progress
- Auto-hide prompt after successful install

Features:
- Automatic install prompt after 3 seconds
- Manual install from settings
- Platform-specific instructions
- Smart dismissal tracking
- Storage info visualization

Closes #35

app/composables/usePWAInstall.ts

@@ -0,0 +1,93 @@
+/**
+ * Composable to handle PWA installation
+ * 
+ * Usage:
+ * const { canInstall, isInstalled, promptInstall, dismissInstall } = usePWAInstall()
+ */
+export function usePWAInstall() {
+  const canInstall = ref(false)
+  const isInstalled = ref(false)
+  const deferredPrompt = ref<any>(null)
+
+  if (process.client) {
+    // Check if already installed
+    if (window.matchMedia('(display-mode: standalone)').matches) {
+      isInstalled.value = true
+    }
+
+    // Listen for beforeinstallprompt event
+    window.addEventListener('beforeinstallprompt', (e) => {
+      // Prevent the mini-infobar from appearing on mobile
+      e.preventDefault()
+      
+      // Stash the event so it can be triggered later
+      deferredPrompt.value = e
+      canInstall.value = true
+    })
+
+    // Listen for appinstalled event
+    window.addEventListener('appinstalled', () => {
+      isInstalled.value = true
+      canInstall.value = false
+      deferredPrompt.value = null
+    })
+  }
+
+  async function promptInstall() {
+    if (!deferredPrompt.value) {
+      return { outcome: 'not-available' }
+    }
+
+    // Show the install prompt
+    deferredPrompt.value.prompt()
+
+    // Wait for the user to respond to the prompt
+    const { outcome } = await deferredPrompt.value.userChoice
+
+    // Clear the deferredPrompt
+    deferredPrompt.value = null
+    
+    if (outcome === 'accepted') {
+      canInstall.value = false
+    }
+
+    return { outcome }
+  }
+
+  function dismissInstall() {
+    canInstall.value = false
+    deferredPrompt.value = null
+    
+    // Remember dismissal for 7 days
+    if (process.client) {
+      localStorage.setItem('pwa-install-dismissed', Date.now().toString())
+    }
+  }
+
+  function shouldShowPrompt() {
+    if (!canInstall.value || isInstalled.value) {
+      return false
+    }
+
+    if (process.client) {
+      const dismissed = localStorage.getItem('pwa-install-dismissed')
+      if (dismissed) {
+        const dismissedTime = parseInt(dismissed)
+        const sevenDays = 7 * 24 * 60 * 60 * 1000
+        if (Date.now() - dismissedTime < sevenDays) {
+          return false
+        }
+      }
+    }
+
+    return true
+  }
+
+  return {
+    canInstall: readonly(canInstall),
+    isInstalled: readonly(isInstalled),
+    promptInstall,
+    dismissInstall,
+    shouldShowPrompt
+  }
+}