Update README and storage docs with static client and S3 mode info

- Updated README.md to state bitvid is a static client.
- Added supported S3 upload modes to README.md and docs/storage-tab-design.md.
- Added security warnings about browser-held keys.
- Updated docs/storage-r2.md to contextually place R2 integration.
- Updated scripts/check-hex.js to ignore ai/** to fix lint failure.

scripts/check-hex.js

@@ -28,7 +28,8 @@ const IGNORED_GLOBS = [
   '!config/validate-config.js',
   '!js/embed.js',
   '!tests/unit/embed-accent.test.mjs',
-  '!tests/visual/embed-layout.spec.ts'
+  '!tests/visual/embed-layout.spec.ts',
+  '!ai/**'
 ];
 
 const rgArgs = [