refactor: Modularize main.rs (2478 → 58 lines), flat architecture
Split monolithic main.rs into domain modules:
- context.rs: AppContext shared state, BgMsg enum, serial_spawn helper
- models.rs: UI model builders, keycap labels, key entries, layout preview
- config.rs: keyboard config export/import via binary protocol
- dispatch.rs: BgMsg handler + WPM/layout timers
- keymap.rs: key selection, layer switch/rename, heatmap toggle
- key_selector.rs: dispatch_keycode router, apply_keycode, hex/MT/LT
- macros.rs: macro CRUD, shortcut presets, step builder
- advanced.rs: combos, KO, leaders, tap dance, BT, tama, autoshift
- settings.rs: OTA flash, config backup, keyboard layout
- flasher.rs: ESP32 bootloader flash
- layout.rs: layout JSON preview, load/export
- connection.rs: serial connect/disconnect, tab auto-refresh
- stats.rs: stats refresh
Rename logic/ → protocol/ with cleaner file names.
Remove unused original-src/ directory.
Fix DarkLineEdit double styling, add rename popup, macro shortcuts.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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