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/components/InstallPrompt.vue

@@ -0,0 +1,104 @@
+<template>
+  <Transition
+    enter-active-class="transition ease-out duration-300"
+    enter-from-class="opacity-0 translate-y-2"
+    enter-to-class="opacity-100 translate-y-0"
+    leave-active-class="transition ease-in duration-200"
+    leave-from-class="opacity-100 translate-y-0"
+    leave-to-class="opacity-0 translate-y-2"
+  >
+    <div
+      v-if="showPrompt"
+      class="fixed bottom-4 left-4 right-4 md:left-auto md:right-4 md:max-w-md z-50"
+    >
+      <div class="bg-white dark:bg-gray-800 rounded-xl shadow-2xl border border-gray-200 dark:border-gray-700 p-4">
+        <div class="flex items-start gap-4">
+          <!-- App Icon -->
+          <div class="flex-shrink-0">
+            <img 
+              src="/icon-192x192.png" 
+              alt="Pantry icon" 
+              class="w-16 h-16 rounded-lg shadow-md"
+            />
+          </div>
+          
+          <!-- Content -->
+          <div class="flex-1 min-w-0">
+            <h3 class="text-lg font-bold text-gray-900 dark:text-white mb-1">
+              Install Pantry
+            </h3>
+            <p class="text-sm text-gray-600 dark:text-gray-400 mb-3">
+              Install this app for quick access and offline use
+            </p>
+            
+            <!-- Actions -->
+            <div class="flex gap-2">
+              <UButton
+                color="emerald"
+                size="sm"
+                @click="install"
+                :loading="installing"
+              >
+                <template #leading>
+                  <UIcon name="i-heroicons-arrow-down-tray" />
+                </template>
+                Install
+              </UButton>
+              
+              <UButton
+                color="gray"
+                variant="ghost"
+                size="sm"
+                @click="dismiss"
+              >
+                Not now
+              </UButton>
+            </div>
+          </div>
+          
+          <!-- Close button -->
+          <button
+            @click="dismiss"
+            class="flex-shrink-0 text-gray-400 hover:text-gray-600 dark:hover:text-gray-300 transition"
+          >
+            <UIcon name="i-heroicons-x-mark" class="w-5 h-5" />
+          </button>
+        </div>
+      </div>
+    </div>
+  </Transition>
+</template>
+
+<script setup lang="ts">
+const { canInstall, promptInstall, dismissInstall, shouldShowPrompt } = usePWAInstall()
+const installing = ref(false)
+const showPrompt = ref(false)
+
+// Show prompt after a delay if conditions are met
+onMounted(() => {
+  setTimeout(() => {
+    if (shouldShowPrompt()) {
+      showPrompt.value = true
+    }
+  }, 3000) // Wait 3 seconds after page load
+})
+
+async function install() {
+  installing.value = true
+  try {
+    const { outcome } = await promptInstall()
+    if (outcome === 'accepted') {
+      showPrompt.value = false
+    }
+  } catch (error) {
+    console.error('Install failed:', error)
+  } finally {
+    installing.value = false
+  }
+}
+
+function dismiss() {
+  dismissInstall()
+  showPrompt.value = false
+}
+</script>