Add Grafana-style time range presets: All, 5m, 15m, 1h, 24h (ticket 024)

LogTimeline gets a preset button row (All + four relative windows)
next to the existing drag-select interaction. App.tsx lifts the
active preset alongside `range`; a relative preset re-resolves to a
fresh {startMs, endMs} on the same 2s cadence the app already polls
on, so it visibly rolls forward rather than freezing at selection
time. Dragging on the timeline (or picking a different preset)
overrides back to a fixed range, matching today's drag behavior.

The rolling re-resolution reads/writes `range` via a ref inside the
poll loop instead of depending on `range` directly, since a naive
`setRange` inside an effect keyed on `range` would restart the effect
(and its interval) every tick — turning "poll every 2s" into "poll as
fast as React can re-render." Verified live: exactly 3 /api/stats
calls over ~6.2s with a rolling preset active, and console/network
free of errors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Docs.tsx

@@ -130,8 +130,14 @@ const SECTIONS: DocSection[] = [
       <div className="flex flex-col gap-2">
         <p>
           The timeline above the log table renders log volume per bucket over the active window. Drag across it to
-          select a fixed time range — it narrows the log table, metrics, and dashboards below to that window; click
-          "Clear selection" to go back to the default (rolling "all time") view.
+          select a fixed time range — it narrows the log table, metrics, and dashboards below to that window.
+        </p>
+        <p>
+          The preset buttons (<Code>All</Code>, <Code>5m</Code>, <Code>15m</Code>, <Code>1h</Code>,{' '}
+          <Code>24h</Code>) are Grafana-style shortcuts for the same thing: <Code>All</Code> clears the range
+          entirely, and a relative preset is <em>rolling</em> — its window keeps advancing to stay "last N minutes
+          from now" rather than freezing at the moment you picked it. Dragging on the timeline while a preset is
+          active switches back to a fixed (non-rolling) range, same as dragging normally would.
         </p>
       </div>
     ),