fix(frontend): percent-encode fileUrl key segments

fileUrl interpolated the raw storage key into the path, so a key segment
containing a reserved character (`?`, `#`, `%`, space) could be
reinterpreted as a query/fragment delimiter. Encode each `/`-separated
segment with encodeURIComponent, keeping slashes as literal path
separators. Keys are backend-generated today, so this is defence-in-depth.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.93.8"
+version = "0.93.9"
 edition = "2021"
 default-run = "mangalord"