Keep image review downloads and external opens distinct

[zrmarley]

Summary

• Keeps proxied relay image review sources on the existing Download image path.
• Routes direct HTTPS image review sources through the shared host onOpenLink plumbing and labels them Open image in browser instead of presenting them as downloads.
• Extracts the existing attachment source policy so file attachments and the image review viewer share the same proxy-vs-external decision.

Linear: https://linear.app/squareup/issue/BOT-1930/view-images-and-galleries

Source-derived risk

• This is a narrow policy/wiring fix, not full gallery validation.
• Download image for relay proxy media and external Open image in browser intentionally remain distinct shared-policy cases.
• ChannelsPage.openLink can return false for ordinary HTTPS by design, so this PR does not claim verified native system-browser external opener behavior. The _blank fallback attributes are established; real browser/package navigation remains pending.
• BOT-1930 remains incomplete for package protected read/native saving work tracked with BOT-2016/BOT-1884. Copy image work is separate in BOT-2022.

Design notes

• Read ~/goose artifacts/buzz-design-system-pack/AGENTS.md, DESIGN.md, and components/registry.json; IconButton is listed with status proposed.
• Reuses the existing repo IconButton and the Phosphor gateway export for the existing ArrowSquareOutIcon.
• No new visual tokens, styles, media files, screenshots, recordings, assets, media/binary fixtures, or downloads were added. tests/fixtures/messages.tsx changed only as code fixture data.
• This follows the existing repository icon policy; the registry/IconButton packaging design conflict remains noted, not changed here.

Tests

• bin/pnpm typecheck
• bin/pnpm exec biome check --error-on-warnings src/bundled/channels/ChannelsPage.tsx src/features/messages/FileAttachment.tsx src/features/messages/ImageReviewStage.tsx src/features/messages/MediaReviewViewer.tsx src/features/messages/attachment-source.ts src/features/messages/MediaReviewViewer.test.tsx src/features/messages/icon-labels.test.tsx tests/fixtures/messages.tsx
• bin/pnpm exec vitest run src/features/messages/MediaReviewViewer.test.tsx src/features/messages/icon-labels.test.tsx — 2 files, 10 tests
• bin/pnpm design:check
• Push hook: TypeScript plus related Vitest selection — 17 files, 220 tests; design-system guards passed
• Slopaganda Panda review: PASS, 2/10 smell, no blockers

Deferred / not verified

• No browser cases were added or removed. The changed contract is covered at the colocated React Testing Library layer because it is component policy/wiring and semantic DOM behavior.
• The only verified end-to-end claim here is the RTL viewer path from media review source policy to host opener callback.
• Real browser navigation, decode/download behavior, packaged/native opener behavior, screenshots/media artifacts, native tests, and a full just scan are deferred pending explicit approval.