feat: add Browser scene, web-element context and relay server improvements

[bobleer]

Summary

• Browser Scene: Add a new embedded Browser scene with Tauri webview integration and element inspector, allowing users to browse web pages directly within BitFun
• Web Element Context: Introduce WebElementContext type and supporting infrastructure, enabling users to reference and share web page elements as context in chat conversations
• Chat Input Improvements: Fix focus management and context tag insertion reliability — activate input before inserting tags, use requestAnimationFrame for focus timing
• Relay Server Hardening: Bind to 127.0.0.1 by default for security, redesign the landing page with modern UI, and fix workspace dependency resolution in Cargo.toml

Changes

Frontend (src/web-ui/)

• New BrowserScene and BrowserPanel components with URL bar, navigation controls, and element inspector
• Register browser scene in scene registry, viewport, and navigation
• Add WebElementContext type and WebElementContextImpl for context system
• Improve ChatInput focus and tag insertion logic
• Add web-element serialization in useMessageSender
• Add i18n keys for browser scene (en-US, zh-CN)

Desktop (src/apps/desktop/)

• Add browser_api.rs with browser_webview_eval and browser_pull_debug_logs commands
• Add webview capabilities in default.json and new browser-webview.json
• Enable Tauri unstable feature for webview API support

Relay Server (src/apps/relay-server/)

• Bind to 127.0.0.1 via RELAY_HOST_BIND_IP env variable for security
• Redesign index.html landing page
• Fix Cargo.toml to use explicit versions instead of broken workspace refs

Mobile Web (src/mobile-web/)

• Update App.tsx with enhanced layout

Test plan

• Verify Browser scene opens from the scene bar and can navigate to URLs
• Test element inspector captures web elements and inserts them as context tags in chat
• Confirm ChatInput properly focuses and inserts context tags when triggered
• Verify relay server starts with start.sh and binds to 127.0.0.1
• Check i18n labels display correctly in both English and Chinese

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