feat(chat): add reply-to threading (M8.3)

Reply button on all messages shows quoted excerpt above input.
On send, quote is prepended as `> excerpt`. Clean up isDark
patterns in ChatInput (dark-only app).

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

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

@@ -6,23 +6,37 @@
       :is-searching="isSearching"
       @select="handleSearchSelect"
     />
+
+    <!-- Reply-to quote -->
+    <div
+      v-if="replyTo"
+      class="mb-2 flex items-start gap-2 rounded-xl bg-white/5 border border-white/10 px-3 py-2 animate-fade-up-fast"
+    >
+      <div class="flex-1 min-w-0">
+        <p class="text-[10px] text-accent/70 font-medium mb-0.5">Replying to</p>
+        <p class="text-xs text-white/50 truncate">{{ replyTo.excerpt }}</p>
+      </div>
+      <button
+        class="shrink-0 w-5 h-5 flex items-center justify-center rounded-md hover:bg-white/10 transition-colors text-white/40 hover:text-white/70"
+        aria-label="Cancel reply"
+        @click="$emit('clearReply')"
+      >
+        <svg class="w-3 h-3" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12" />
+        </svg>
+      </button>
+    </div>
+
     <div
       class="path-glass-bubble rounded-2xl px-4 py-3 flex items-end gap-2 transition-all duration-300"
-      :class="focused
-        ? isDark
-          ? 'border-white/30'
-          : 'border-black/20'
-        : ''"
+      :class="focused ? 'border-white/30' : ''"
     >
       <textarea
         ref="textareaRef"
         v-model="text"
         rows="1"
         :placeholder="placeholder"
-        class="flex-1 resize-none bg-transparent text-sm outline-none min-h-[24px] max-h-[120px]"
-        :class="isDark
-          ? 'text-white/90 placeholder:text-white/25'
-          : 'text-gray-800 placeholder:text-gray-400'"
+        class="flex-1 resize-none bg-transparent text-sm outline-none min-h-[24px] max-h-[120px] text-white/90 placeholder:text-white/25"
         @keydown.enter.exact.prevent="send"
         @input="autoResize"
         @paste="onPaste"
@@ -44,8 +58,7 @@
         <button
           v-if="hasPasted && canSend"
           :disabled="!canSend"
-          class="shrink-0 path-glass-button path-glass-button-sm rounded-xl px-3 transition-all duration-200 hover:opacity-80 active:scale-95"
-          :class="isDark ? 'text-accent/80' : 'text-accent'"
+          class="shrink-0 path-glass-button path-glass-button-sm rounded-xl px-3 transition-all duration-200 hover:opacity-80 active:scale-95 text-accent/80"
           aria-label="Extract content"
           title="Contextualize — extract media without sending to AI"
           @click="extract"
@@ -75,7 +88,6 @@
 
 <script setup lang="ts">
 import { ref, computed, nextTick, watch } from 'vue'
-import { useTheme } from '@/composables/useTheme'
 import { useFederatedSearch, type SearchResult } from '@/composables/useFederatedSearch'
 import SearchResults from '@/components/ui/SearchResults.vue'
 
@@ -84,11 +96,13 @@ const props = withDefaults(
     disabled?: boolean
     streaming?: boolean
     placeholder?: string
+    replyTo?: { messageId: string; excerpt: string } | null
   }>(),
   {
     disabled: false,
     streaming: false,
     placeholder: 'Message AIUI...',
+    replyTo: null,
   }
 )
 
@@ -96,9 +110,9 @@ const emit = defineEmits<{
   send: [text: string]
   extract: [text: string]
   stop: []
+  clearReply: []
 }>()
 
-const { isDark } = useTheme()
 const text = ref('')
 const focused = ref(false)
 const hasPasted = ref(false)

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

@@ -27,6 +27,13 @@
         >
           <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
+          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="Reply"
+          @click.stop="$emit('reply', message.id, message.content)"
+        >
+          <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="M7.707 3.293a1 1 0 010 1.414L5.414 7H11a7 7 0 017 7v2a1 1 0 11-2 0v-2a5 5 0 00-5-5H5.414l2.293 2.293a1 1 0 11-1.414 1.414l-4-4a1 1 0 010-1.414l4-4a1 1 0 011.414 0z" 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"
@@ -275,6 +282,7 @@ const emit = defineEmits<{
   edit: [messageId: string, newContent: string]
   regenerate: []
   branch: [messageId: string]
+  reply: [messageId: string, content: string]
 }>()
 
 // Editing state

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

@@ -62,6 +62,7 @@
               @edit="handleEdit"
               @regenerate="handleRegenerate"
               @branch="handleBranch"
+              @reply="handleReply"
             />
           </ErrorBoundary>
         </div>
@@ -76,9 +77,11 @@
       :disabled="isStreaming"
       :streaming="isStreaming"
       :placeholder="isStreaming ? 'Waiting for response...' : 'Message AIUI...'"
+      :reply-to="replyTo"
       @send="handleSend"
       @extract="handleExtract"
       @stop="handleStop"
+      @clear-reply="clearReply"
     />
   </div>
 </template>
@@ -123,6 +126,18 @@ import { useCodeContext } from '@/composables/useCodeContext'
 const codeContext = useCodeContext()
 const messageListRef = ref<HTMLElement | null>(null)
 
+// Reply-to threading state
+const replyTo = ref<{ messageId: string; excerpt: string } | null>(null)
+
+function handleReply(messageId: string, content: string) {
+  const excerpt = content.slice(0, 100) + (content.length > 100 ? '...' : '')
+  replyTo.value = { messageId, excerpt }
+}
+
+function clearReply() {
+  replyTo.value = null
+}
+
 const messages = computed(() => chatStore.messages)
 
 const virtualizer = useVirtualizer(computed(() => ({
@@ -244,7 +259,14 @@ async function handleSend(text: string) {
     }
   }
 
-  await sendMessage(text)
+  // Prepend quote if replying to a message
+  let finalText = text
+  if (replyTo.value) {
+    const quoteLine = replyTo.value.excerpt.split('\n').map(l => `> ${l}`).join('\n')
+    finalText = `${quoteLine}\n\n${text}`
+    clearReply()
+  }
+  await sendMessage(finalText)
 }
 
 function handlePromptSelect(_userMsg: Message, assistantMsg: Message | null) {