Custom time range and brush/zoom for Status charts

The Status time picker is presets-only (1h/6h/24h/7d/30d), which is limiting for incident forensics ("what happened Tuesday 2–4pm?"). Two related additions that share plumbing:

1. A calendar/date-time custom range in `TimeRangePicker` — slots into the existing `timeRange` context; auto-refresh should disable for historical windows.
2. Brush/zoom on the time-series charts (Recharts ``, or drag-select → narrow `timeRange`), so a spike seen at 7d can be zoomed into without guessing the preset.

## Acceptance criteria
- `TimeRangePicker` gains a "Custom…" option opening a date+time range picker; selecting one sets the window and forces auto-refresh to Off while active (a historical window must not slide).
- Deep-linkable: encodes as `range=custom&start=&end=` in the status route's search schema (`statusSearch.ts`), with validation (start < end, span capped at 30d, invalid → default preset).
- Brush/drag-select on any time-series chart narrows to that sub-range (becomes a custom range); a "back to preset" affordance restores the previous preset.
- `bucketMs` for custom windows derives from span using the same clamp rationale as `TIME_PRESETS` (see the SRE note atop `timePresets.ts` — do not allow 1m buckets on a 30d span).

## Implementation notes
- Extend `validateStatusSearch` (start/end only meaningful when `range=custom`); `StatusTabsInner`'s ctx memo branches on preset vs custom.
- Check `src/components/ui/` for an existing shadcn calendar before adding one.

Filed from a Status tab code review.

Comment generated by kAIle (Claude Fable 5)