feat(mobile): move reader CSS application from Rust to frontend for HMR support
Add get_reader_content command that returns article content + metadata as JSON.
Add show_inline_html command that displays pre-built HTML in native WKWebView.
App.tsx now imports reader.css via Vite ?raw, builds the full reader HTML template
in the frontend, and passes it to show_inline_html. This means reader.css changes
are picked up by HMR without requiring a Rust rebuild.
The old open_inline_webview_html command is preserved but no longer called.