Move brand images to proprietary/ and support multi-dir asset pipeline (#3662)

## Description:

* Move proprietary brand images (logos, favicon) from resources/images/
to proprietary/images/ to separate open-source assets from proprietary
ones
* Extend the asset pipeline (PublicAssetManifest, vite.config.ts) to
support multiple source directories (resources/ + proprietary/), so
buildAssetUrl resolves assets from either location transparently
* In dev, serve proprietary/ as a fallback middleware (registered after
Vite's publicDir handler) so resources/ takes precedence when files
exist in both. The idea is we could have placeholder assets placeholders
that can be used by forks, and only the production build uses
proprietary assets.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

tests/client/sound/SoundManager.test.ts

@@ -39,17 +39,6 @@ vi.mock("howler", () => {
   return { Howl: MockHowl };
 });
 
-// Mock music imports
-vi.mock("../../../../proprietary/sounds/music/of4.mp3", () => ({
-  default: "of4.mp3",
-}));
-vi.mock("../../../../proprietary/sounds/music/openfront.mp3", () => ({
-  default: "openfront.mp3",
-}));
-vi.mock("../../../../proprietary/sounds/music/war.mp3", () => ({
-  default: "war.mp3",
-}));
-
 // Mock the Sounds module so tests don't depend on actual asset paths
 vi.mock("../../../src/client/sound/Sounds", async (importOriginal) => {
   const actual =