(#54) Remove global enable alerts setting

[bfalls]

Fixes #54

• Removed the notifications onboarding step and its callback so the flow moves directly from location selection to alarm configuration.
• Simplified onboarding state to rely solely on per-alarm enablement when loading and saving settings.
• Updated Android permission prompts to trigger from per-alarm toggles instead of a global notification switch.
• Doing this step by step, more onboarding steps to be removed.