feat(extension): P2 视频页悬浮按钮 + 右键菜单 + cookie 直通；P3 侧边栏首版

- contentScripts: 仅在支持的视频平台（B 站 / YouTube / 抖音 / 快手）注入悬浮 BiliNote 按钮，点击通过 webext-bridge 发 'bilinote-start' 给 background
- background: 处理 bilinote-start 与右键菜单点击；调 /api/generate_note；写 chrome.storage；自动打开侧边栏。logic/storage 是 Vue 反应式版本，service worker 不能 import，因此把常量抽到 logic/constants.ts
- contextMenus: onInstalled 时注册"用 BiliNote 总结此视频"，限定 4 个支持平台域名
- 浏览器 Cookie 同步：options 页加按钮，按平台读 chrome.cookies.getAll，序列化为 'name=value; ...' 后 POST 给后端 /api/update_downloader_cookie。chrome.cookies + contextMenus 权限补到 manifest
- 侧边栏（P3 首版）：从 storage 读最近任务并轮询，复用 TaskProgress + MarkdownView。markmap 思维导图与 RAG 问答推到后续
- 修 P1 endpoint 拼错的 bug：/api/get_models_by_provider 实际是 /api/model_enable，404 来自这里

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

BillNote_extension/src/contentScripts/views/App.vue

@@ -1,9 +1,57 @@
 <script setup lang="ts">
-// P2 计划：在视频页注入悬浮按钮 → 一键调起 BiliNote 任务。
-// MVP 阶段无注入；保留组件外壳以便编译与未来扩展。
 import 'uno.css'
+import { computed, ref } from 'vue'
+import { sendMessage } from 'webext-bridge/content-script'
+import { detectPlatform, PLATFORM_LABELS } from '~/logic/platform'
+
+const platform = detectPlatform(window.location.href)
+const busy = ref(false)
+const toast = ref<{ kind: 'ok' | 'err', text: string } | null>(null)
+
+const label = computed(() => platform ? `用 BiliNote 总结这个${PLATFORM_LABELS[platform]}视频` : '')
+
+async function trigger() {
+  if (!platform || busy.value)
+    return
+  busy.value = true
+  toast.value = null
+  try {
+    const res = await sendMessage('bilinote-start', {
+      url: window.location.href,
+      platform,
+    }, 'background')
+    const ok = res && (res as any).ok
+    toast.value = ok
+      ? { kind: 'ok', text: '已开始生成笔记，可在侧边栏 / popup 查看进度' }
+      : { kind: 'err', text: (res as any)?.error || '提交失败，请打开设置检查后端与供应商' }
+  }
+  catch (e) {
+    toast.value = { kind: 'err', text: (e as Error).message }
+  }
+  finally {
+    busy.value = false
+    setTimeout(() => { toast.value = null }, 4000)
+  }
+}
 </script>
 
 <template>
-  <!-- intentionally empty in P1 MVP -->
+  <div v-if="platform" class="bilinote-fab fixed bottom-24 right-6 z-[2147483647] flex flex-col items-end gap-2 font-sans select-none">
+    <div
+      v-if="toast"
+      class="text-xs px-3 py-2 rounded shadow max-w-[260px]"
+      :class="toast.kind === 'ok' ? 'bg-green-600 text-white' : 'bg-red-600 text-white'"
+    >
+      {{ toast.text }}
+    </div>
+    <button
+      class="flex items-center gap-2 px-3 py-2 rounded-full shadow-lg cursor-pointer border-none text-white text-sm font-medium bg-pink-600 hover:bg-pink-700 disabled:bg-pink-300"
+      :disabled="busy"
+      :title="label"
+      @click="trigger"
+    >
+      <span class="text-base">📝</span>
+      <span>{{ busy ? '提交中…' : 'BiliNote' }}</span>
+    </button>
+  </div>
 </template>