Two bugs in the Overview tab Packets/Hour chart:
1. **Bars not rendering**: `barW` went negative when `data.length` was
large (e.g. 720 hours for 30-day range), producing zero-width invisible
bars. Fix: `Math.max(1, ...)` floor on bar width.
2. **X-axis labels overlapping**: Every single hour label was emitted
(`02h03h04h...`). Fix: decimate labels based on time range — every 6h
for ≤24h, every 12h for ≤72h, every 24h beyond. Shows `MM-DD` on
midnight boundaries for multi-day ranges.
**Scope**: Only touches the Overview tab `Packets / Hour` section and
the shared `barChart` floor (one-line change). No modifications to
Topology, Channels, Distance, or other tabs.
Fixes#858
Co-authored-by: you <you@example.com>
ea78581eea
