Reliability: Add technical failure fallback for sensor-based challenges

[knocker-g]

Summary

Introduces a fail-safe mechanism for sensor-based dismiss challenges (e.g., shake, squats) when a technical failure or sensor unavailability is detected.

Problem

In rare cases where a device sensor fails or becomes unavailable while an alarm is firing, a user might find themselves stuck in a challenge with no way to dismiss the alarm.

Changes

• Added a fallback path in AlarmFiringActivity that triggers only upon a detected technical failure (e.g., sensor not found).
• Ensured this fallback is separate from normal challenge solving; it cannot be used to bypass a challenge that is functioning correctly.
• Added unit tests for the fallback trigger logic in AlarmFiringViewModelTest.kt. Automated test execution is currently blocked by existing environment issues.

Testing

• git diff --check: PASS
• F-Droid debug build: SUCCESS
• Play debug build: SUCCESS
• Tests: BLOCKED / NOT EXECUTED
• Manual verification: Simulated sensor unavailability to confirm the fallback path activates correctly. Verified that normal challenge solving is unaffected.