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.

templates/side/elements/navigation.htm

@@ -9,6 +9,8 @@
   <ul id="nav-list">
     <li><a href="{{ url_for('side_main.index') }}">NanoShare</a></li>
 
+    <li><a href="{{ url_for('link_composer.link_composer_page') }}">Links</a></li>
+
     {% if user %}
       <li><a href="{{ url_for('side_main.index') }}">Upload</a></li>
       <li><a href="{{ url_for('side_main.files_list') }}">Files</a></li>