feat: v0.4 — satellite imagery, KiwiSDR radio, LOCATE bar & security cleanup

New features:
- NASA GIBS (MODIS Terra) daily satellite imagery with 30-day time slider
- Esri World Imagery high-res satellite layer (sub-meter, zoom 18+)
- KiwiSDR SDR receivers on map with embedded radio tuner
- Sentinel-2 intel card — right-click for recent satellite photo popup
- LOCATE bar — search by coordinates or place name (Nominatim geocoding)
- SATELLITE style preset in bottom bar cycling
- v0.4 changelog modal on first launch

Fixes:
- Satellite imagery renders below data icons (imagery-ceiling anchor)
- Sentinel-2 opens full-res PNG directly (not STAC catalog JSON)
- Light/dark theme: UI stays dark, only map basemap changes

Security:
- Removed test files with hardcoded API keys from tracking
- Removed .git_backup directory from tracking
- Updated .gitignore to exclude test files, dev scripts, cache files

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

frontend/src/lib/ThemeContext.tsx

@@ -0,0 +1,39 @@
+"use client";
+
+import React, { createContext, useContext, useState, useEffect } from "react";
+
+type Theme = "dark" | "light";
+
+const ThemeContext = createContext<{ theme: Theme; toggleTheme: () => void }>({
+  theme: "dark",
+  toggleTheme: () => {},
+});
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setTheme] = useState<Theme>("dark");
+
+  useEffect(() => {
+    const saved = localStorage.getItem("sb-theme") as Theme | null;
+    if (saved === "light" || saved === "dark") {
+      setTheme(saved);
+      document.documentElement.setAttribute("data-theme", saved);
+    }
+  }, []);
+
+  const toggleTheme = () => {
+    const next = theme === "dark" ? "light" : "dark";
+    setTheme(next);
+    localStorage.setItem("sb-theme", next);
+    document.documentElement.setAttribute("data-theme", next);
+  };
+
+  return (
+    <ThemeContext.Provider value={{ theme, toggleTheme }}>
+      {children}
+    </ThemeContext.Provider>
+  );
+}
+
+export function useTheme() {
+  return useContext(ThemeContext);
+}