feat(electrobun): settings overhaul — fonts, shells, providers, retention, chords
- Settings drawer: responsive width clamp(24rem, 45vw, 50rem)
- System font detection: fc-list for UI fonts (preferred sans-serif starred)
and mono fonts (Nerd Fonts starred), fallback to hardcoded lists
- Scrollback: default 5000, min 1000, step 500
- Shell detection: system.shells RPC, pre-selects $SHELL login shell
- Provider enablement: provider.scan gates toggle, unavailable shown as N/A
- Session retention: count 0-100 (0=Keep all), age 0-365 (0=Forever)
- Chord keybindings: Ctrl+K → Ctrl+S style multi-key sequences,
1s prefix wait, arrow separator display, 26 tests passing
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