refactor(settings): remove dead machine path fields from frontend

Slice 2 of jellyfin-service-registry. Removes the machine-level
media_root/path_prefix fields from the frontend now that the backend no
longer stores them.

- types/index.ts: dropped media_root/path_prefix from MonitoringMachine
  and MonitoringMachineInput.
- pages/Settings.tsx: removed the media_root form input, the read-only
  "Media root" detail (replaced with a local-hint field mirroring the
  editor), and media_root/path_prefix from emptyMachine() and both
  edit-handler reset mappings; updated the section description.
- tests: removed media_root/path_prefix from Settings/Media/FileBrowser
  test fixtures.

npm run build (tsc -b + vite) clean; 0 lint errors; 72 tests pass.

frontend/src/pages/__tests__/Media.test.tsx

@@ -32,8 +32,6 @@ function machineFixture(
 		ssh_private_key_set: false,
 		ssh_private_key_passphrase_set: false,
 		password_set: false,
-		media_root: "",
-		path_prefix: "",
 		notes: "",
 		...overrides,
 	};