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

src/client/sound/SoundManager.ts

@@ -1,7 +1,5 @@
 import { Howl } from "howler";
-import of4 from "../../../proprietary/sounds/music/of4.mp3";
-import openfront from "../../../proprietary/sounds/music/openfront.mp3";
-import war from "../../../proprietary/sounds/music/war.mp3";
+import { assetUrl } from "../../core/AssetUrls";
 import { EventBus } from "../../core/EventBus";
 import { UserSettings } from "../../core/game/UserSettings";
 import {
@@ -33,19 +31,19 @@ export class SoundManager {
     this.safely("initialize background music", () => {
       this.backgroundMusic = [
         new Howl({
-          src: [of4],
+          src: [assetUrl("sounds/music/of4.mp3")],
           loop: false,
           onend: this.playNext.bind(this),
           volume: 0,
         }),
         new Howl({
-          src: [openfront],
+          src: [assetUrl("sounds/music/openfront.mp3")],
           loop: false,
           onend: this.playNext.bind(this),
           volume: 0,
         }),
         new Howl({
-          src: [war],
+          src: [assetUrl("sounds/music/war.mp3")],
           loop: false,
           onend: this.playNext.bind(this),
           volume: 0,