fix(android): open external links in the system browser

[guivieiras]

Android link clicks previously reached an open_url stub that only logged a warning. They now launch the device’s browser or associated app through ACTION_VIEW.

This reuses the existing Java UI-thread bridge, catches ActivityNotFoundException, and updates the platform and design docs. No new dependencies or UI controls.

Validation:

• Passed cargo fmt --all --check, cargo build --workspace --locked, and cargo test --workspace --locked on the topic branch.
• Passed RUSTFLAGS='-D warnings' cargo ndk -t arm64-v8a check -p tcode-android --locked and Java compilation.
• Built and signature-verified a release APK from integration containing this fix. The user verified HTTP/HTTPS links, query parameters, returning to Tcode, and Preview’s external-browser action on Android.
• Workspace Clippy failed on existing nonminimal_bool and collapsible_match warnings in unchanged runtime/UI code. Android lint reported three existing errors in the keyboard bridge and styles. cargo machete was unavailable.

No new unit test was added for the native bridge wiring; device testing covers the interaction. The no-handler fallback was not exercised on a device.