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/base_n.py

@@ -0,0 +1,28 @@
+def encode_int(value: int, alphabet: str) -> str:
+  if value < 0:
+    raise ValueError("cannot encode negative integers")
+  if value == 0:
+    return alphabet[0]
+
+  base = len(alphabet)
+  out = []
+  while value:
+    value, digit = divmod(value, base)
+    out.append(alphabet[digit])
+  return "".join(reversed(out))
+
+def decode_int(text: str, alphabet: str) -> int:
+  index = {char: i for i, char in enumerate(alphabet)}
+  value = 0
+  base = len(alphabet)
+  for char in text:
+    if char not in index:
+      raise ValueError(f"invalid character for alphabet: {char!r}")
+    value = value * base + index[char]
+  return value
+
+def bytes_to_int(data: bytes) -> int:
+  return int.from_bytes(data, "big") if data else 0
+
+def int_to_bytes(value: int, length: int) -> bytes:
+  return value.to_bytes(length, "big") if length else b""