feat(forum): add bottom navigation bar

5 tabs at the bottom of the forum standalone:
  Posts (→ /forum), Spaces, Submit (→ /submit), Search, Alerts

Spaces, Search, and Alerts are dimmed and emit a "coming soon" toast
on tap pointing at the tracking issue:
  - Spaces  → #31 (NIP-72 communities)
  - Search  → #15 (link aggregator search)
  - Alerts  → #32 (per-standalone notifications, hub aggregation)

Mirrors the activities-app bottom-bar pattern (icon + 10px label,
fixed bottom, safe-area-aware) and replaces the previous bare
forum-app shell which had no way to compose a new submission.

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

src/forum-app/App.vue

@@ -7,7 +7,9 @@ import { useTheme } from '@/components/theme-provider'
 import { toast } from 'vue-sonner'
 import { useAuth } from '@/composables/useAuthService'
 import { Button } from '@/components/ui/button'
-import { LogIn } from 'lucide-vue-next'
+import {
+  LogIn, Newspaper, Hash, SquarePen, Search, Bell,
+} from 'lucide-vue-next'
 
 const route = useRoute()
 const router = useRouter()
@@ -17,8 +19,39 @@ const { isAuthenticated } = useAuth()
 
 const showLoginDialog = ref(false)
 
+interface Tab {
+  name: string
+  icon: any
+  path?: string
+  comingSoon?: { issue: number; label: string }
+}
+
+const bottomTabs: Tab[] = [
+  { name: 'Posts', icon: Newspaper, path: '/forum' },
+  { name: 'Spaces', icon: Hash, comingSoon: { issue: 31, label: 'Spaces' } },
+  { name: 'Submit', icon: SquarePen, path: '/submit' },
+  { name: 'Search', icon: Search, comingSoon: { issue: 15, label: 'Search' } },
+  { name: 'Alerts', icon: Bell, comingSoon: { issue: 32, label: 'Notifications' } },
+]
+
 const isLoginPage = computed(() => route.path === '/login')
 
+function isActiveTab(tab: Tab): boolean {
+  if (!tab.path) return false
+  if (tab.path === '/forum') return route.path === '/forum' || route.path.startsWith('/submission/')
+  return route.path.startsWith(tab.path)
+}
+
+function onTabClick(tab: Tab) {
+  if (tab.path) {
+    router.push(tab.path)
+  } else if (tab.comingSoon) {
+    toast.info(`${tab.comingSoon.label} — coming soon`, {
+      description: `Tracked on issue #${tab.comingSoon.issue}`,
+    })
+  }
+}
+
 async function handleLoginSuccess() {
   showLoginDialog.value = false
   toast.success('Welcome!')
@@ -36,9 +69,33 @@ async function handleLoginSuccess() {
         </Button>
       </div>
 
-      <main class="flex-1">
+      <main class="flex-1" :class="{ 'pb-16': !isLoginPage }">
         <router-view />
       </main>
+
+      <nav
+        v-if="!isLoginPage"
+        class="fixed bottom-0 left-0 right-0 z-50 border-t bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/60"
+        style="padding-bottom: env(safe-area-inset-bottom)"
+      >
+        <div class="flex items-center justify-around h-14 max-w-lg mx-auto">
+          <button
+            v-for="tab in bottomTabs"
+            :key="tab.name"
+            class="flex flex-col items-center justify-center gap-0.5 flex-1 h-full transition-colors"
+            :class="[
+              isActiveTab(tab)
+                ? 'text-primary'
+                : 'text-muted-foreground hover:text-foreground',
+              tab.comingSoon ? 'opacity-50' : '',
+            ]"
+            @click="onTabClick(tab)"
+          >
+            <component :is="tab.icon" class="w-5 h-5" />
+            <span class="text-[10px] font-medium">{{ tab.name }}</span>
+          </button>
+        </div>
+      </nav>
     </div>
 
     <Toaster />