v0.8.0: POTUS fleet tracking, full aircraft color-coding, carrier fidelity, UI overhaul

New features:
- POTUS fleet (AF1, AF2, Marine One) with hot-pink icons + gold halo ring
- 9-color aircraft system: military, medical, police, VIP, privacy, dictators
- Sentinel-2 fullscreen overlay with download/copy/open buttons (green themed)
- Carrier homeport deconfliction — distinct pier positions instead of stacking
- Toggle all data layers button (cyan when active, excludes MODIS Terra)
- Version badge + update checker + Discussions shortcut in UI
- Overhauled MapLegend with POTUS fleet, wildfires, infrastructure sections
- Data center map layer with ~700 global DCs from curated dataset

Fixes:
- All Air Force Two ICAO hex codes now correctly identified
- POTUS icon priority over grounded state
- Sentinel-2 no longer overlaps bottom coordinate bar
- Region dossier Nominatim 429 rate-limit retry/backoff
- Docker ENV legacy format warnings resolved
- UI buttons cyan in dark mode, grey in light mode
- Circuit breaker for flaky upstream APIs

Community: @suranyami — parallel multi-arch Docker builds + runtime BACKEND_URL fix (PR #35, #44)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Former-commit-id: 7c523df70a2d26f675603166e3513d29230592cd

.env.example

@@ -0,0 +1,16 @@
+# ShadowBroker — Docker Compose Environment Variables
+# Copy this file to .env and fill in your keys:
+#   cp .env.example .env
+
+# ── Required for backend container ─────────────────────────────
+OPENSKY_CLIENT_ID=
+OPENSKY_CLIENT_SECRET=
+AIS_API_KEY=
+
+# ── Optional ───────────────────────────────────────────────────
+
+# LTA (Singapore traffic cameras) — leave blank to skip
+# LTA_ACCOUNT_KEY=
+
+# Override the backend URL the frontend uses (leave blank for auto-detect)
+# NEXT_PUBLIC_API_URL=http://192.168.1.50:8000