fix(nav): correct proguard keep rule to use extends instead of implements

Proguard/R8 uses extends for both subclassing and interface implementation, so the implements keyword was invalid.

app/proguard-rules.pro

@@ -49,5 +49,5 @@
 
 # ── Navigation (NavKey Names) ─────────────────────────────────────────────
 # Keep screen object class names because their simpleNames are used for bottom nav selection and storage.
--keep class * implements androidx.navigation3.runtime.NavKey { *; }
+-keep class * extends androidx.navigation3.runtime.NavKey { *; }