fix: make regular keypress sounds audible on Pixel

[limeimetw]

Summary

• route regular Android keypress sounds to the audible spacebar effect instead of Pixel/AOSP's near-silent standard effect
• add an androidTest regression for regular-key sound mapping at 100% keypress volume
• update 打字震動功能無效 #128 tracking docs/backlog with Pixel vibration-ok / sound-follow-up status

Test Plan

• cd LimeStudio && ./gradlew :app:compileDebugJavaWithJavac :app:compileDebugAndroidTestJavaWithJavac
• cd LimeStudio && ./gradlew :app:assembleDebug

Device Verification Needed

• Pixel / Android 17: vibration remains OK, regular character keys are now as audible as Backspace / Space / Enter
• Samsung A17: stronger vibration levels and keypress_sound_volume preference still behave as expected

Follow-up for #128.

[limeimetw]

Closing this outdated PR because the key-sound work is being fixed elsewhere.