chore(fc2a): polish pass — toasts, loading skeletons, focus guards, mobile breakpoints

AppSnackbar mounts once at the app root and exposes a window.__fcToast
function the stores call from error paths. This avoids prop-drilling a
toast emitter through every component while keeping the snackbar
component itself testable in isolation.

GalleryGrid renders shimmer-skeleton placeholders on initial load so the
first paint isn't empty. Skeleton uses the same auto-fill grid columns
as the real items so layout doesn't shift when content arrives.

Modal arrow nav now ignores text-entry elements so typing in the tag
autocomplete doesn't navigate prev/next. Small-screen breakpoint
(<600px) tightens gallery thumbnails to 120px min.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/AppSnackbar.vue

@@ -0,0 +1,26 @@
+<template>
+  <v-snackbar
+    v-model="show" :color="color" location="bottom right" timeout="4000"
+    multi-line elevation="4"
+  >
+    {{ message }}
+    <template #actions>
+      <v-btn variant="text" @click="show = false">Dismiss</v-btn>
+    </template>
+  </v-snackbar>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+const show = ref(false)
+const message = ref('')
+const color = ref('error')
+
+function open({ text, type = 'error' }) {
+  message.value = text
+  color.value = type === 'error' ? 'error' : type === 'success' ? 'success' : 'info'
+  show.value = true
+}
+
+defineExpose({ open })
+</script>