feat: add self-contained link composer

- Add structured URL codec with compact alphabets and dictionary support.
- Add public decode, redirect, and QR endpoints for composed links.
- Add authenticated encode API and link composer UI.
- Generate PNG QR codes via qrcode with Pillow support.
- Register the new blueprint and navigation entry.
- Cover public decode, auth gating, redirect URL parsing, and QR output.
- Bump NanoShare to 1.27.0 and lock new dependencies.

requirements.txt

@@ -41,6 +41,7 @@ pycparser==3.0
 pygments==2.19.2
 pyjwt==2.12.1
 python-dotenv==1.2.2
+qrcode[pil]>=8.0
 quart==0.20.0
 quart-flask-patch==0.3.0
 -e ./quart-session