feat(qr): decode QR codes from an uploaded image

[lyfuci]

What

The QR tool could only generate codes — this adds the missing reverse half: decode a QR from an image.

A Generate / Decode view toggle sits at the top of the page. The Decode view shows a drop-zone (reusing FileDrop) that accepts an image, rasterises it on an offscreen canvas, and runs jsQR entirely locally — nothing is uploaded. The decoded payload renders with a copy button, plus an "open link" affordance when it parses as an http(s) URL (the regex is anchored, so javascript: etc. can't slip through). The "no QR found" and non-image-file branches surface an inline error.

How

• src/lib/qr-decode.ts
  • decodeQrFromImageData(data, w, h) — thin wrapper over jsQR on raw RGBA; no canvas/network, so it's unit-testable in Node.
  • decodeQrFromBlob(blob) — browser path: createImageBitmap -> canvas -> getImageData -> decode, with a downscale cap for large images.
• src/pages/QrCode.tsx — view toggle + decode panel; generator UI unchanged, just wrapped in the generate branch.
• New dep: jsqr (pure-JS, MIT).
• i18n keys added in both en.json and zh-CN.json.

Tests / verification

• New round-trip unit test: render a QR via the qrcode lib -> rasterise -> decodeQrFromImageData -> assert the original text (URL + CJK cases), plus a blank-image -> null case.
• pnpm typecheck / pnpm lint / pnpm build clean; 211 tests green.
• Headless-Chrome run against a fresh build: upload a QR PNG in the Decode view -> decoded text + working open-link appear, no raw i18n keys, zero page errors; the Generate view still renders its SVG.

🤖 Generated with Claude Code