Move browser demo static assets to public/ for Vite (#3215)

The Vite migration removed the copy-assets.js script but didn't
move audio/ and image/ files into public/ (Vite's publicDir).
This caused speech.mp3, speech_stereo.mp3, and beach.jpg to be
missing from dist/ after build.

- Move audio/*.mp3 and image/*.jpg to public/
- Remove /public/ from .gitignore (was preventing tracking)
- Consolidate REFERENCES.md with all attribution info
- Remove now-empty audio/ and image/ directories

Author: xuesichao

demos/browser/.gitignore

@@ -1,2 +1 @@
 /debug.js
-/public/

demos/browser/audio/REFERENCES.md

@@ -0,0 +1,5 @@
+File | Source
+---- | ------
+speech.mp3 | Excerpt from speech by Barack Obama, November 20, 2014. Source: The White House. President Obama delivers an address to the nation on immigration. https://millercenter.org/the-presidency/presidential-speeches/november-20-2014-address-nation-immigration
+speech_stereo.mp3 | Audio test file for stereo verification
+beach.jpg | Licensed under the Creative Commons Attribution 2.0 Generic license. Author: Familydestinationsguide.com Images. https://commons.wikimedia.org/wiki/File:Hotels_Lined_up_Along_a_Beach_Illustration.jpg

demos/browser/image/license.html

@@ -48,6 +48,13 @@ function runCommandWithLogs(command) {
   }
 }
 
+// If the output directory was cleaned but the tsbuildinfo persists, tsc --build
+// will incorrectly skip compilation. Delete the tsbuildinfo in that case.
+const tsBuildInfoPath = `${configDir}/tsconfig.mediatransformworker.tsbuildinfo`;
+if (!fs.existsSync('./build/mediatransformworker') && fs.existsSync(tsBuildInfoPath)) {
+  fs.unlinkSync(tsBuildInfoPath);
+}
+
 // Transpile media transform worker TypeScript files.
 // Note: prebuild runs via npm lifecycle before build, so node_modules is already available
 let tscPath = path.resolve(`${execSync('npm root').toString().trim()}/.bin/tsc`);