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.

link_composer/qr.py

@@ -0,0 +1,19 @@
+from io import BytesIO
+
+import qrcode
+from qrcode.constants import ERROR_CORRECT_M
+
+def make_qr_png(data: str) -> bytes:
+  qr = qrcode.QRCode(
+    version=None,
+    error_correction=ERROR_CORRECT_M,
+    box_size=10,
+    border=4,
+  )
+  qr.add_data(data, optimize=20)
+  qr.make(fit=True)
+  image = qr.make_image(fill_color="black", back_color="white")
+
+  buffer = BytesIO()
+  image.save(buffer, format="PNG")
+  return buffer.getvalue()