feat: add factory reset functionality

[b4nst]

Summary

Hold Setup button for 3 seconds to reset all settings to factory defaults.

Closes #22

Changes

• Add warning LED patterns (medium red at 1s, full red at 2s)
• Add success pattern (green at 3s)
• Add flash_factory_reset() function
• Track setup button hold time in timer event
• Use volatile for shared timer/interrupt variables
• Fix FlashData structure alignment for ARM Cortex-M3

Reset Sequence

1. 1 second: Grid shows medium red warning
2. 2 seconds: Grid shows full red warning
3. 3 seconds: Reset executes, green success pattern
4. 500ms later: Returns to normal display

Behavior

• Short press (< 1s): Normal setup mode toggle (unchanged)
• Release during warning: Cancels reset, returns to previous display
• Hold 3+ seconds: Factory reset executes

Test plan

• Short press still toggles setup mode
• 1 second hold shows medium red warning
• 2 second hold shows full red warning
• 3 second hold executes reset and shows green
• Release during warning cancels and refreshes display
• All pages reset to defaults
• Flash updated with defaults
• Unit tests pass (97 tests)
• ARM build succeeds

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