Polish transcript copy control and terminal landmark

[jim80net]

What this fixes

• Makes Copy transcript read as a clear, compact desktop action while preserving a full-width control at 390 px.
• Removes the duplicated terminal landmark when the final transcript passage is already short enough to read in place; long endings retain a useful excerpt.

Walk evidence

The keyless, GPU-less 2026-08-15 walk against exact main 0c646a646423c4eed83f56aafdf2d989bcea5655 reproduced both issues in Light and Dark pixels at 1440 px and 390 px. The same 286-segment YouTube transcript was then rendered from this branch for the landing check:

• W1: the Copy transcript control is compact on desktop and full width at 390 px in both themes.
• W2: the terminal landmark is “End,” and “All right thank you Lisha.” remains once as the final paragraph; no duplicated terminal section is rendered.
• All four widths/themes report scrollWidth == viewportWidth.

Validation

• uv run pytest — 1,198 passed, 1 skipped
• uv run ruff check src/podcast_reader/html.py tests/test_html.py
• uv run mypy src
• npm test — 453 passed
• npm run typecheck
• npm run lint
• npx playwright test tests/e2e/artifact-geometry.spec.ts --project=e2e — 26 passed

This PR does not perform signing, notarization, release, or paid operations. It is intentionally left unmerged for independent review.

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Summary by cubic

Polishes the transcript export control and terminal landmark naming. Desktop now shows a compact “Copy transcript” button (was a bordered full‑width block); narrow screens keep a full‑width control. The final timeline marker uses “End” for short closing passages; long endings still show an excerpt.

• CSS: moves border/shadow/background to the .transcript-export-panel; .transcript-export becomes a grid with justify-items: end and no chrome; the toggle is auto‑width and centered on desktop; @media (max-width: 900px) stretches the toggle to full width and left‑aligns it.
• Timeline: adds _timeline_marker_label and uses it in the rail and in‑flow landmarks to avoid echoing short terminal passages.
• Tests: adds a Playwright regression test for control geometry across widths/themes; adds unit tests for the CSS and terminal landmark behavior; updates HTML fixtures.

^{Written for commit 1ef27ba. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 9 files

_{Re-trigger cubic}

[jim80net]

XO re-gate PASS at exact head 1ef27ba636fd95f1cc6da0cf310d2ba3ec4f1e65.

Prior format block fixed (tests/test_html.py only). Hosted quality/macos-build/app-e2e/frozen-smoke green. cubic 0 inline comments. Named-ref only. No signing/notarize/release/spend.