feat(chat): add context window visualiser bar (M8.6)

Slim progress bar showing estimated token usage (~4 chars/token).
Bitcoin-orange fill turns red when >80% of context window used.
Tooltip shows exact token estimate on hover.

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

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

@@ -18,6 +18,8 @@
       @scroll-to-message="scrollToMessageIndex"
     />
 
+    <ContextBar :messages="messages" />
+
     <!-- Collapsed: prompt index -->
     <PromptIndex
       v-if="chatCollapsed"
@@ -105,6 +107,7 @@ import StreamingDots from './StreamingDots.vue'
 import PromptIndex from './PromptIndex.vue'
 import BranchSwitcher from './BranchSwitcher.vue'
 import ChatSearch from './ChatSearch.vue'
+import ContextBar from './ContextBar.vue'
 import ErrorBoundary from '@/components/ui/ErrorBoundary.vue'
 import type { Message } from '@aiui/core/types/message'
 

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

@@ -0,0 +1,52 @@
+<template>
+  <div
+    v-if="messages.length > 0"
+    class="h-1 mx-3 rounded-full bg-white/5 overflow-hidden shrink-0 group cursor-help relative"
+    :title="tooltipText"
+  >
+    <div
+      class="h-full rounded-full transition-all duration-500"
+      :class="percentage > 80 ? 'bg-red-500' : 'bg-accent'"
+      :style="{ width: `${Math.min(percentage, 100)}%` }"
+    />
+    <!-- Tooltip on hover -->
+    <div
+      class="absolute -top-8 left-1/2 -translate-x-1/2 hidden group-hover:flex items-center px-2 py-1 rounded-md bg-black/80 text-[10px] text-white/80 whitespace-nowrap pointer-events-none z-10"
+    >
+      {{ tooltipText }}
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed } from 'vue'
+import type { Message } from '@aiui/core/types/message'
+
+const props = defineProps<{
+  messages: Message[]
+  contextWindow?: number
+}>()
+
+// Default context window sizes per model (tokens)
+const maxTokens = computed(() => props.contextWindow ?? 200000)
+
+// Estimate: ~4 chars per token
+const estimatedTokens = computed(() => {
+  let chars = 0
+  for (const msg of props.messages) {
+    chars += msg.content.length
+  }
+  return Math.ceil(chars / 4)
+})
+
+const percentage = computed(() => {
+  if (maxTokens.value === 0) return 0
+  return (estimatedTokens.value / maxTokens.value) * 100
+})
+
+const tooltipText = computed(() => {
+  const est = estimatedTokens.value.toLocaleString()
+  const max = maxTokens.value.toLocaleString()
+  return `~${est} / ${max} tokens used`
+})
+</script>