feat(android): bottom tab bar (Connect / Settings)

Replace the ad-hoc screen switching with a Material3 bottom NavigationBar. Two top-level
destinations — Connect (Home icon) and Settings (gear) — persist across tab switches; the
immersive stream view is shown full-screen, outside the bar. Settings is now a tab, so its
button is dropped from the Connect screen.

- app/build.gradle.kts: + androidx.compose.material:material-icons-core (tab icons).
- MainActivity: Screen sealed interface -> Tab enum; App() wraps the tabs in a Scaffold with a
  NavigationBar bottomBar (streamHandle != 0 -> StreamScreen full-screen); ConnectScreen drops
  the onOpenSettings param + the Settings button.

Verified live (emulator): the bar renders with Connect/Settings; tapping a tab swaps content and
moves the selected indicator; the bar persists on both tabs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/android/app/build.gradle.kts

@@ -59,6 +59,7 @@ dependencies {
     implementation("androidx.compose.ui:ui-tooling-preview")
     implementation("androidx.compose.foundation:foundation")
     implementation("androidx.compose.material3:material3")
+    implementation("androidx.compose.material:material-icons-core") // bottom-bar tab icons
     debugImplementation("androidx.compose.ui:ui-tooling")
 
     // Android TV components (we target phone + TV) land in the TV-UI milestone: