Initial commit: Share-as-QR Firefox extension with bun build

MV3 WebExtension that turns the active tab's URL into a scannable QR
code via a toolbar popup. The popup runs locally — no network requests.

Build pipeline: `bun build` bundles popup.js + the vendored
kazuhikoarase/qrcode-generator (MIT, pinned to 83b7e8f) into a single
~23 KB minified ESM file under dist/. web-ext operates on dist/, so
the packaged zip contains only what actually ships (~28 KB).

Scripts:
- bun run build    — bundle + copy assets into dist/
- bun run lint     — build + web-ext lint (0 errors / 0 warnings)
- bun run package  — build + produce a signable .zip
- bun run start    — build + launch Firefox with the extension loaded

Tracks dogcat epic firefox-share-as-qr-35mw and its 5 child tasks
(scaffold, vendor lib, popup UI, icons, README).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/build.mjs

@@ -0,0 +1,29 @@
+import { rm, cp } from "node:fs/promises";
+
+const DIST = "dist";
+
+await rm(DIST, { recursive: true, force: true });
+
+const result = await Bun.build({
+  entrypoints: ["popup/popup.js"],
+  outdir: `${DIST}/popup`,
+  target: "browser",
+  format: "esm",
+  minify: true,
+});
+
+if (!result.success) {
+  for (const log of result.logs) console.error(log);
+  process.exit(1);
+}
+
+await Promise.all([
+  cp("manifest.json", `${DIST}/manifest.json`),
+  cp("popup/popup.html", `${DIST}/popup/popup.html`),
+  cp("popup/popup.css", `${DIST}/popup/popup.css`),
+  cp("icons", `${DIST}/icons`, { recursive: true }),
+  cp("vendor/LICENSE.qrcode-generator", `${DIST}/LICENSE.qrcode-generator`),
+]);
+
+const size = (await Bun.file(`${DIST}/popup/popup.js`).arrayBuffer()).byteLength;
+console.log(`Built ${DIST}/ (popup.js: ${(size / 1024).toFixed(1)} KB)`);