Document the extension and add MIT LICENSE

User-verified that v0.1.0 works in Firefox. Closing out the work with
the documentation the AMO reviewer (and future-me) will want:

- LICENSE: MIT, 2026. Matches the vendored qrcode-generator's license.
- package.json: add license + author fields.
- README: add "What you see" UX section, supported-versions rationale,
  ASCII build-flow diagram, AMO publishing notes, permissions table.
- popup/popup.js: top-of-file block documenting the popup lifecycle and
  the dataflow from active-tab URL to rendered QR.
- scripts/build.mjs: top-of-file block explaining why dist/ is the
  single source of truth for what ships.

Closes dogcat epic firefox-share-as-qr-35mw and all 5 child tasks
(see .dogcats/issues.jsonl).

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

package.json

@@ -3,6 +3,8 @@
   "version": "0.1.0",
   "private": true,
   "description": "Firefox extension: share the current tab's URL as a QR code.",
+  "license": "MIT",
+  "author": "Ole-Morten Duesund",
   "scripts": {
     "build": "bun run scripts/build.mjs",
     "lint": "bun run build && bunx --bun web-ext lint",