fix(link-previews): proxy sent preview media

[tellaho]

Overview

Category: fix
User Impact: Sent link previews now reliably display their thumbnail and favicon when the media is hosted on the relay.

Problem: Sent preview cards loaded relay-hosted snapshot media directly, so authenticated relay requests could fail even though the snapshot itself was valid. Solution: Rewrite snapshot media at the shared card render boundary through Buzz's authenticated local media proxy, preserving the original display domain and rerendering when the proxy becomes ready.

Changes

File changes

desktop/src/shared/ui/link-preview-attachment.tsx
Routes sent preview thumbnails and favicons through authenticated relay media handling above the Compact/Rich fork while preserving original metadata.

desktop/src/testing/e2eBridge.ts
Adds an opt-in proxy-readiness seam that deterministically re-arms the production media lookup when released.

desktop/tests/e2e/messaging.spec.ts
Covers the real send, snapshot, recipient, and card-render path for Compact and Rich previews, including fallback URLs, proxied URLs, and decoded image content.

desktop/tests/helpers/bridge.ts
Exposes the opt-in media-proxy startup state to E2E tests.

Reproduction Steps

1. Send a link whose preview snapshot includes a relay-hosted thumbnail and favicon.
2. Inspect the sent message card in Compact mode and confirm both images render after the local media proxy becomes ready.
3. Switch link previews to Rich mode and confirm the thumbnail and favicon continue to render.
4. Run the focused Playwright regression:
   pnpm exec playwright test tests/e2e/messaging.spec.ts --project=smoke --grep "sent link preview media uses the authenticated proxy"

Before / After

Before	After
Relay-hosted preview media fails to load.	The sent preview thumbnail and favicon render through the authenticated media proxy.