feat(link-preview): frontend interface for Telegram-style URL preview

Adds the front-end side of the link-preview feature so the back-end
team has a fixed contract to implement against.

- docs/link-preview.md: full spec for the `/api/link-preview` proxy
  and the preferred inline-on-Post integration. Covers caching, SSRF
  guards, metadata-extraction precedence, provider quirks, and the
  front-end rendering rules. Scope is the first URL only.
- types/post.ts: new `LinkPreview` type and optional `linkPreview`
  field on `Post`.
- LinkPreviewCard: clickable card with a themeColor accent bar,
  siteName / title / description (line-clamped), and an optional
  1.91:1 thumbnail. Whole card is an `<a target="_blank">` to the
  canonical URL.
- MessageBubble: render the card between the bubble body and the
  timestamp, with padding that matches visual vs. text-only bubbles.
- mockPosts: example `linkPreview` payloads on p-005 and p-010 so
  the visual works when running with VITE_USE_MOCK_POSTS=true,
  and so the back-end has concrete reference values.

src/mocks/mockPosts.ts

@@ -156,6 +156,19 @@ export const MOCK_POSTS: Post[] = [
     isRecommended: false,
     publishedAt: "2026-01-19T16:20:00.000Z",
     updatedAt: "2026-01-19T16:20:00.000Z",
+    // Mock: only the FIRST URL in the text is previewed.
+    linkPreview: {
+      url: "https://coinmarketcap.com/currencies/ark-defai/",
+      canonicalUrl: "https://coinmarketcap.com/currencies/ark-defai/",
+      siteName: "coinmarketcap.com",
+      title: "ARK DeFAI Price, Chart & Market Cap",
+      description:
+        "Track ARK DeFAI live price, market cap, volume and historical chart on CoinMarketCap. Verified contract address, holders and on-chain analytics.",
+      imageUrl: img(81, 1200, 630),
+      imageWidth: 1200,
+      imageHeight: 630,
+      themeColor: "#2962FF",
+    },
   },
 
   // 6) 纯文本 + 单链接（简短公告）
@@ -254,6 +267,15 @@ export const MOCK_POSTS: Post[] = [
     categorySlug: "meeting",
     language: "zh-CN",
     text: "📌 ARK DeFAI 方舟晨间时刻\n\n🧠 会议主题：市场概况交流 & 市场问题讨论。\n🕙 会议时间：3月1日（日）10:00\n🎬 直播腾讯会议链接：https://meeting.tencent.com/l/G718S4Sedm38",
+    linkPreview: {
+      url: "https://meeting.tencent.com/l/G718S4Sedm38",
+      canonicalUrl: "https://meeting.tencent.com/l/G718S4Sedm38",
+      siteName: "meeting.tencent.com",
+      title: "腾讯会议 · ARK DeFAI 方舟晨间时刻",
+      description:
+        "点击直接加入直播会议。需要 App 或浏览器插件。会议号会在点击后自动补全。",
+      themeColor: "#0080FF",
+    },
     attachments: [
       {
         id: "a-010",