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>

popup/popup.js

@@ -1,3 +1,20 @@
+// Popup entry point. Runs every time the user clicks the toolbar icon.
+//
+// Lifecycle:
+//   1. `init()` reads the active tab's URL via the WebExtension `tabs` API
+//      (granted by the `activeTab` permission, which only activates on user
+//      gesture — no persistent host access).
+//   2. Browser-internal schemes (about:, moz-extension:, ...) short-circuit
+//      to a friendly "can't share this page" message.
+//   3. Otherwise `renderQr()` asks the vendored qrcode-generator for an SVG
+//      (smallest version that fits, error-correction level M ~15%) and
+//      mounts it into the popup.
+//   4. The Copy URL button uses navigator.clipboard.writeText, gated by the
+//      `clipboardWrite` permission.
+//
+// The vendored library is imported here so `bun build` can inline it; there
+// is no global `qrcode` at runtime.
+
 import qrcode from "../vendor/qrcode.js";
 
 // Schemes that either can't be QR-encoded usefully or are browser-internal.