feat(chat): add conversation branching UI (M8.2)

Wire up branch-from-message handler, add BranchSwitcher glass pill
component showing "Branch X of Y" with prev/next navigation.
Clean up isDark conditionals in chat components (dark-only app).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/src/components/chat/ChatMessage.vue

@@ -27,6 +27,14 @@
         >
           <svg xmlns="http://www.w3.org/2000/svg" class="w-3.5 h-3.5" viewBox="0 0 20 20" fill="currentColor"><path fill-rule="evenodd" d="M4 2a1 1 0 011 1v2.101a7.002 7.002 0 0111.601 2.566 1 1 0 11-1.885.666A5.002 5.002 0 005.999 7H9a1 1 0 010 2H4a1 1 0 01-1-1V3a1 1 0 011-1zm.008 9.057a1 1 0 011.276.61A5.002 5.002 0 0014.001 13H11a1 1 0 110-2h5a1 1 0 011 1v5a1 1 0 11-2 0v-2.101a7.002 7.002 0 01-11.601-2.566 1 1 0 01.61-1.276z" clip-rule="evenodd" /></svg>
         </button>
+        <button
+          v-if="!isUser"
+          class="glass-button-sm !h-7 !min-h-0 !w-7 !min-w-0 !px-0 flex items-center justify-center text-white/60 hover:text-white/90"
+          title="Branch from here"
+          @click.stop="$emit('branch', message.id)"
+        >
+          <svg xmlns="http://www.w3.org/2000/svg" class="w-3.5 h-3.5" viewBox="0 0 20 20" fill="currentColor"><path fill-rule="evenodd" d="M5 3a2 2 0 00-2 2v2a2 2 0 002 2h2a2 2 0 002-2V5a2 2 0 00-2-2H5zM5 11a2 2 0 00-2 2v2a2 2 0 002 2h2a2 2 0 002-2v-2a2 2 0 00-2-2H5zM11 5a2 2 0 012-2h2a2 2 0 012 2v2a2 2 0 01-2 2h-2a2 2 0 01-2-2V5z" clip-rule="evenodd" /></svg>
+        </button>
       </div>
 
       <div
@@ -241,7 +249,6 @@ import { computed, ref, nextTick } from 'vue'
 import MarkdownIt from 'markdown-it'
 import type { Message, WebSearchResult } from '@aiui/core/types/message'
 import type { Film, Song, Podcast, Book, TVSeries, ImageItem, Place } from '@aiui/core/types/content'
-import { useTheme } from '@/composables/useTheme'
 import { useContentPanel, type MagazineSection } from '@/composables/useContentPanel'
 import { useCodeContext } from '@/composables/useCodeContext'
 import FilmCard from '@/components/content/FilmCard.vue'
@@ -267,10 +274,9 @@ const props = withDefaults(
 const emit = defineEmits<{
   edit: [messageId: string, newContent: string]
   regenerate: []
+  branch: [messageId: string]
 }>()
 
-const { isDark } = useTheme()
-
 // Editing state
 const isEditing = ref(false)
 const editContent = ref('')