chore(tui): visual polish — detail sections, column headers, alert detail (#37)
## Summary
Bundled remaining UX polish items from the screenshot review.
### Changes
**Detail panel sections (#5)**
- Fields grouped into ENDPOINT, TIMING, HTTP, CONFIG sections with subtle headers
- Matches existing PROBE RESULTS and STATE CHANGES section pattern
- Cleaner visual hierarchy without box-drawing clutter
**Omit unconfigured fields (#6)**
- Timeout hidden when 0 (unconfigured)
- Method hidden when default GET
- AcceptedCodes shows "200-299" explicitly when empty
**Column header (#7)**
- `LATENCY` → `LAT` (design short, never truncate — htop/btop pattern)
**Alert detail view (#8)**
- `i` key on Alerts tab opens full detail panel
- Shows: type, health status, last sent time, send/fail counts, last error
- Full config key:value pairs (untruncated)
- Keybinding: `[i/Esc] Back [e] Edit [t] Test [q] Quit`
### Files (3)
- `internal/tui/tab_sites.go` — section headers, field omission, LAT header
- `internal/tui/tab_alerts.go` — viewAlertDetailPanel()
- `internal/tui/tui.go` — stateAlertDetail, key handler, render routing
Reviewed-on: lerko/uptop#37
lerko
cfbf01274d