fix(review): visual-capture table renders oversized mobile thumbnails and never gives up on a missing preview build

[JSONbored]

Summary

Two bugs observed live in the "Visual preview" PR comment table on a real repo review:

1. Mobile screenshots are oversized/undownscaled. capturePage's display-thumbnail generation
   (src/review/visual/capture.ts) is gated to viewportName === "desktop" only. The mobile capture
   is 390px wide — already under the display-downscale width cap — but shot.ts captures
   fullPage: true, so a mobile screenshot's HEIGHT is just as unbounded as desktop's. The result: a
   several-thousand-pixel-tall full-page mobile capture renders at native size in the comment table
   instead of a bounded thumbnail. Separately, downscaleForDisplay
   (src/selfhost/stubs/image-downscale.ts) only bounds width (no fit/height), so even removing the
   desktop-only gate alone would not have fixed this for a narrow-but-tall image.

2. The "after" cell never resolves out of the loading placeholder for a repo with no discoverable
   preview-build signal. buildCapture's preview-build-state check
   (src/review/visual/capture.ts) only applies the poll-budget give-up logic
   (preview-poll-budget.ts, MAX_PREVIEW_POLL_ATTEMPTS) when getPreviewBuildState returns
   "building" or "succeeded". When it returns "absent" (no Workers-Builds-named check-run found
   at all — the case for a repo whose frontend has no preview-deploy CI configured), nothing happens:
   previewPending/previewFailed both stay false, so the "Rendering preview…" spinner placeholder
   is shown and never re-evaluated to a terminal state on any subsequent review pass.

Expected behavior

• The mobile row's thumbnail is bounded on both width and height, matching the desktop row's existing
  behavior.
• When no preview-build signal is ever found (and actions_fallback isn't configured to fill the
  gap), the "after" cell eventually shows the honest "Preview deploy failed — review manually" card
  after a bounded number of poll attempts, instead of spinning forever.

Validation

• npm run test:coverage (unsharded) on the affected files, both new/updated branches covered.
• npm run test:ci full local gate green.

[linear-code]

JSO-5120