Chrome extension port (stage 3)

[perdrizat]

After the AMO release (3.0.0) bakes, port NewTab PowerTools to Chrome on top of the chrome-prep program (2.5.0). References issue #15.

The chrome-prep program (see CONTRIBUTING.md "Decisions of record" and git history/CHANGELOG.md for the completed C0–C6 arcs) deliberately reduced this port to a bounded, reviewable set of changes:

What's already done (prerequisites, chrome-prep program)

• Background DOM-guard, typed monoliths, feature-module split, the api capability seam (webextension/api.js page-side, webextension/lib/platform.js background-side — a live Proxy over globalThis.browser ?? chrome), and two-target manifest authoring (manifest/base.json + manifest/firefox.json + manifest/chrome.json, merged by scripts/build-manifest.mjs).
• Full divergence audit: audit/2026-07-11-chrome-api-divergence.md (29 portable surfaces, 6 divergent-wrap, 4 Firefox-only). The 6 wrappers (storage.session, capture-availability, search-shape, action/theme-icons, menus presence-gate, getBrowserInfo short-circuit) are implemented and Firefox-verified, but their Chrome-side branches are written and dormant — no Chrome manifest has ever exercised them.
• Two design decisions settled up front and already encoded in the seam:
  • Decision 1 (context menus): Chrome ships WITHOUT dynamic context menus — the in-tile action row (edit/never-capture/pin/remove) is the Chrome interaction. menus registration is already presence-gated on api.menus in both lib/background-main.js and newTab.js.
  • Decision 2 (theme): prefers-color-scheme is the base theme source on both platforms; browser.theme is a Firefox-only bonus layered on top. syncActionIconWithTheme is already a Chrome-dormant no-op stub.

What's left (this issue)

1. Fork the seam implementations. The api Proxy and its 6 wrappers already branch on globalThis.browser ?? chrome; verify each Chrome branch against a real Chrome install (currently unvalidated beyond "the code compiles/typechecks").
2. Write the OffscreenCanvas thumbnail-image.js. lib/thumbnail-image.js is the one file in lib/ allowed to touch DOM/canvas (guarded by the C1 ESLint rule) — Chrome's MV3 service worker has no DOM, so the capture pipeline needs an OffscreenCanvas-based sibling implementation, selected via the same capability-seam pattern.
3. Activate the Chrome manifest overlay. manifest/chrome.json exists and pnpm build chrome already stages and zips a dormant build (dist/*-chrome.zip) — this issue is about validating it against a real Chrome install and unretiring it from "dormant."
4. New test surface. Decide how Chrome gets covered by the test tiers (a second E2E/UAT target is a real design question — the current tiers are Firefox-only by design) before this ships.

Non-goals

• Long-lived Chrome branch — explicitly rejected (CONTRIBUTING.md "Decisions of record": single-source/dual-build, not parallel branches).
• The previous maintainer's chrome branch is historical reference only — do not merge it.