Build shared browser UI as an npm project

[AgentStart1]

The shared browser UI previously shipped manually vendored dependencies, so dependency upgrades required copying assets into Android resources. Move it into an independent npm project under web/ and build its distribution automatically for every Android variant.

• Manage Markdown, icons, and build/test tools through package.json and package-lock.json; enable weekly Dependabot npm updates.
• Bundle JS/CSS with esbuild, copy HTML and referenced icons, and generate third-party license notices from bundled packages.
• Register generated Java resources through the AGP variant API so Gradle installs locked dependencies and packages the web distribution into APKs.
• Configure Node.js 24 in test/release workflows, run browser tests against the built distribution, and verify APK resources against that distribution. No CI locking was added.
• Document the npm workflow and Node.js 24 build prerequisite.

Validation:

• 12 web tests and the Chromium browser regression suite passed.
• 63 JVM tests passed, including real Ktor route and generated-resource checks.
• Debug, Alpha, and Release builds passed; Alpha/Release ran R8. Earlier combined runs lost the Gradle daemon; separate retries completed successfully.
• All 15 APKs contain the expected 16 web resources, matching the npm distribution byte-for-byte.
• Android device runtime testing was not performed for this change.