fix: return NotFound when a storage key resolves to a directory

`get` (fs::read → EISDIR) and `get_stream` (File::open succeeds on a Unix
directory, then streams garbage that fails mid-read) both mishandled a key
resolving to a directory. Mirror `size`'s existing guard: `get` maps
IsADirectory to NotFound; `get_stream` checks metadata.is_file() after open.

Test: get_on_directory_is_not_found.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.128.6",
+  "version": "0.128.7",
   "private": true,
   "type": "module",
   "scripts": {