fix(sidecar): probe installed and dev paths for the binary

dpkg-deb -c on the bundled .deb shows mycelium at /usr/bin/mycelium
(no triple suffix), next to the app launcher. Our previous resolver
only looked for the suffixed dev name in resource_dir, so the
installed app could not find its sidecar.

Probe order is now: directory of current_exe, then resource_dir,
then CARGO_MANIFEST_DIR/binaries, then $PATH, with both the plain
"mycelium" name and the dev-style "mycelium-<triple>" alias
checked at each location.

src-tauri/tauri.conf.json

@@ -29,7 +29,7 @@
   "bundle": {
     "active": true,
     "targets": ["deb", "appimage"],
-    "category": "Network",
+    "category": "Utility",
     "shortDescription": "Mycelium overlay network client",
     "longDescription": "Desktop GUI for the Mycelium end-to-end encrypted IPv6 overlay network.",
     "icon": [