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.

routes/__init__.py

@@ -18,6 +18,7 @@ auth_login_bp = create_auth_login_blueprint(
 from .side.main import side_main_bp
 
 from .side.upload import upload_bp
+from .side.link_composer import link_composer_bp
 
 from .api.cli_auth import cli_auth_bp
 from .api.download import api_download_bp