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

frontend/Dockerfile

@@ -10,13 +10,17 @@ FROM base AS builder
 WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
-ENV NEXT_TELEMETRY_DISABLED 1
+ENV NEXT_TELEMETRY_DISABLED=1
+# NEXT_PUBLIC_* vars must exist at build time for Next.js to inline them.
+# Default empty = auto-detect from browser hostname at runtime.
+ARG NEXT_PUBLIC_API_URL=""
+ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
 RUN npm run build
 
 FROM base AS runner
 WORKDIR /app
-ENV NODE_ENV production
-ENV NEXT_TELEMETRY_DISABLED 1
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
 
 RUN addgroup --system --gid 1001 nodejs
 RUN adduser --system --uid 1001 nextjs
@@ -32,7 +36,7 @@ COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
 USER nextjs
 
 EXPOSE 3000
-ENV PORT 3000
-ENV HOSTNAME "0.0.0.0"
+ENV PORT=3000
+ENV HOSTNAME="0.0.0.0"
 
 CMD ["node", "server.js"]