feat(15-03): add signal history charts with expandable rows in station and link tables

- Create SignalHistoryChart with recharts LineChart, green/yellow/red reference bands, and 24h/7d/30d range selector
- Add expandable rows to WirelessStationTable (click station to see signal history)
- Add expandable rows to WirelessLinksTable CPE rows (click link to see signal history)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.planning/ROADMAP.md

@@ -117,7 +117,7 @@ Plans:
   2. System detects and surfaces signal degradation trends (e.g., "signal dropped 8dB over 2 weeks")
   3. Operator can create site-scoped alert rules (e.g., "alert when >20% of devices at this site go offline")
   4. Operator can create sector-scoped alert rules (e.g., "alert when sector average signal drops below -75dBm")
-**Plans:** 1/3 plans executed
+**Plans:** 2/3 plans executed
 
 Plans:
 - [ ] 15-01-PLAN.md — Backend data model, services, and REST API for site alert rules, alert events, and signal history
@@ -131,7 +131,7 @@ Plans:
 | Sites | SITE-01, SITE-02, SITE-03, SITE-04, SITE-05, SITE-06 | 11 | 3/3 | Complete    | 2026-03-19 | DASH-01 | 11 | 1 |
 | Site Dashboard | DASH-02, DASH-03, DASH-04 | 14 | 3/3 | Complete    | 2026-03-19 | SECT-01, SECT-02, SECT-03 | 14 | 3 |
 | Wireless Collection | WRCL-01, WRCL-02, WRCL-03, WRCL-04, WRCL-05, WRCL-06 | 12 | 2/2 | Complete    | 2026-03-19 | LINK-01, LINK-02, LINK-03, LINK-04 | 13 | 3/3 | Complete    | 2026-03-19 | WRUI-01, WRUI-02, WRUI-03 | 14 | 3 |
-| Signal Trending | TRND-01, TRND-02 | 15 | 1/3 | In Progress|  | ALRT-01, ALRT-02 | 15 | 2 |
+| Signal Trending | TRND-01, TRND-02 | 15 | 2/3 | In Progress|  | ALRT-01, ALRT-02 | 15 | 2 |
 | **Total** | | | **30** |
 
 ## Progress