Add Chrome browser support and configurable default browser

[pxlarified]

Adds Chrome as a first-class browser alongside Zen, and lets you pick the browser used when --browser is omitted via config browser <browser>.

Chrome support

Chrome joins the adapter registry through a new Chromium-family adapter, mirroring the existing Firefox-family design. The native bridge (native-host.cjs) is already browser-agnostic, so the Chromium adapter reuses it and only differs where Chromium genuinely differs from Firefox:

• Native-messaging manifest uses allowed_origins (chrome-extension://<id>/) instead of Firefox's allowed_extensions, and is written into Chrome's NativeMessagingHosts directory (plus the HKCU\Software\Google\Chrome\NativeMessagingHosts registry key on Windows).
• Chromium can't side-load a packed extension from a profile, so install chrome stages an unpacked extension under ~/OpenBrowser/extensions/chrome/ for "Load unpacked".

To make the native-messaging allowed_origins deterministic, the built Chromium manifest ships a fixed public key, so the unpacked extension always loads with the same extension id. A test recomputes the id from the key to guard the constant.

Shared native-host / process-detection helpers were extracted from firefox-family.js into browsers/shared.js so both families reuse them without duplication (Zen install is unchanged).

The build now emits an MV3 Chromium artifact (dist/extensions/chromium/) from the same extension source; the two extension scripts got a tiny, feature-detected compatibility layer so one source runs on both families:

// background.js / content.js
if (typeof browser === "undefined") var browser = chrome;
// ...
if (browser.scripting?.executeScript) { /* MV3 */ } else { /* MV2 */ }

Configurable default browser

config browser <browser> persists a default to ~/OpenBrowser/config.json. Resolution order is --browser flag, then the configured default, then Zen.

OpenBrowser config browser chrome   # set
OpenBrowser config browser          # show current
OpenBrowser config                  # show full config

Validation

• npm test (17 tests incl. new config + Chrome adapter coverage), npm run build, and web-ext lint on the Firefox source all pass in the sandbox.
• Smoke-tested install zen (regression) and install chrome end to end: native host launcher, native-messaging manifest, and staged extension all land correctly.

Note: the Chrome extension runtime could not be exercised against a live Chrome in CI (no browser available). The CLI, adapter, native-messaging, config, and build layers are unit-tested; loading the unpacked extension in a real Chrome is the remaining manual check.

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Summary by cubic

Adds Chrome as a first-class browser alongside Zen and introduces a default-browser setting so commands can run without passing --browser. Also unifies the extension code across Firefox and Chromium with a shim that prefers the browser API and falls back to chrome.

• New Features

  • Chrome support via a new Chromium-family adapter; installer writes the native-messaging manifest and stages an unpacked extension at ~/OpenBrowser/extensions/chrome/ with a stable ID.
  • MV3 Chromium build added; background/content scripts now prefer browser with a chrome fallback and detect MV2/MV3. Screenshot uses captureVisibleTab when per-tab capture isn’t available.
  • config browser <browser> persists a default in ~/OpenBrowser/config.json; CLI resolves --browser, then the configured default, then Zen.
  • Build updates: npm run build now builds both families; added build:chromium. Version bumped to 2.1.0 across package and manifests.

• Migration

  • Chrome: run install chrome, then in chrome://extensions enable Developer mode and “Load unpacked” from ~/OpenBrowser/extensions/chrome/.
  • Optional: set a default browser with OpenBrowser config browser chrome to omit --browser.

^{Written for commit c26da99. Summary will update on new commits.}

[pxlarified]

/codesmith review

[blacksmith-sh]

@pxlarified

/codesmith review

I reviewed the PR and found one real bug, which I fixed and pushed (c26da99).

Findings

Bug (fixed): content.js cross-browser shim broke the Firefox path. The shim if (typeof browser === "undefined") var browser = chrome; lived inside the content-script IIFE. Because var browser is hoisted to function scope, typeof browser was always "undefined" at the check, so it unconditionally rebound to chrome even in Firefox/Zen. That silently switched the content script off the promise-based browser API it was written against (onMessage listeners returning Promise.resolve(...)), on the path that was previously working. I verified this empirically. background.js had the same idiom but at top-level, where it was actually correct, so the two were inconsistent.

Fixed both to a hoist-safe form that genuinely prefers browser:

const browser = globalThis.browser || globalThis.chrome; // background.js
var   browser = globalThis.browser || globalThis.chrome; // content.js (inside IIFE)

Confirmed: Firefox (both globals present) resolves to browser; Chrome (only chrome) falls back correctly. Rebuilt artifacts, web-ext lint clean (0/0/0), all 17 tests pass.

Other notes (not blocking, no change made)

• Chrome MV3 service worker lifetime. background.js keeps a persistent native-messaging port. An MV3 service worker can be suspended; the reconnect logic handles a dropped port, and an open connectNative port extends SW lifetime, so it should hold, but this is the part most likely to need real-Chrome validation (already flagged in the PR body).
• Stale signed Firefox artifact. dist/extensions/firefox/openbrowser.xpi is still the 2.0.1 signed build (signing needs AMO creds and happens at release), so it lags the source. This is pre-existing repo behavior; sign-firefox regenerates it on release. The install flow prefers the signed xpi, so Zen installs the last released extension until then, which is harmless here.
• build.js reads CHROME_EXTENSION_KEY from constants.js via regex rather than importing it. It's guarded by the id-derivation test and tolerant of whitespace, but an import would be sturdier. Minor.

Everything else (Chromium adapter, native-messaging allowed_origins + registry layout, config resolution order, registry wiring, deterministic extension id) checked out against the intended behavior.