feat(desktop): enrich link previews with page metadata

[tellaho]

Category: improvement
User Impact: Messages now show richer, more recognizable previews for supported HTTPS links.

Problem: Link cards relied on URL-derived or sender-entered labels, so they often lacked the page context users needed. Generic HTTPS links had no compact preview at all.

Solution: Fetch bounded Open Graph, Twitter, or HTML title metadata through the native networking boundary, while preserving provider-specific labels and safe URL fallbacks. Generic links use a bundled globe icon, and unsafe or unavailable metadata fails closed without changing the visible link.

File changes

desktop/src-tauri/src/commands/link_preview.rs
Adds hardened, bounded page-metadata retrieval and title/site-name extraction for public HTTPS URLs.

desktop/src-tauri/src/lib.rs
Registers the native link-preview metadata command.

desktop/src/features/communities/useCommunityInit.ts
Clears ephemeral preview metadata when the active community changes.

desktop/src/shared/lib/linkPreview.test.mjs
Covers generic HTTPS previews, exclusions, ordering, deduplication, and title fallbacks.

desktop/src/shared/lib/linkPreview.ts
Recognizes generic public HTTPS links while retaining specialized provider previews and existing limits.

desktop/src/shared/lib/useResolvedLinkPreviews.ts
Resolves fetched metadata through an ephemeral, deduplicated client cache and merges it into card models.

desktop/src/shared/ui/link-preview-attachment.tsx
Uses the bundled globe as the neutral source mark for generic web previews.

Reproduction steps

1. Run Buzz Desktop and open a channel containing a GitHub, Linear, or Google link; confirm its existing provider label remains and its title resolves from page metadata when available.
2. Send a public generic https:// URL and confirm a compact card appears below the visible original link with a globe source icon.
3. Send an http:// URL and confirm Buzz leaves the link visible without generating a preview card.
4. Switch communities and confirm previews resolve normally without carrying cached metadata across communities.

Screenshots

Generic source icon before/after: