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>

vendor/LICENSE.qrcode-generator

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2009 Kazuhiko Arase
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.