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.
