Wire WiFi creds via .env + build-time injection, add heartbeat reporting

Adopts the weather-display pattern: .env file parsed by tools/load_env.py
and injected as -D compiler flags. WIFI_NETWORKS uses "SSID:password" format.
ESP32 now sends a heartbeat POST after each display update so the server
can track frame status for the HA integration.

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

firmware/tools/load_env.py

@@ -0,0 +1,27 @@
+"""
+PlatformIO pre-build script: reads .env and injects values as -D build flags.
+
+Handles quoting so that comma-separated WIFI_NETWORKS works as a single string.
+"""
+
+Import("env")
+import os
+
+env_file = os.path.join(env.get("PROJECT_DIR", "."), ".env")
+if not os.path.exists(env_file):
+    print("WARNING: .env file not found, skipping credential injection")
+else:
+    with open(env_file) as f:
+        for line in f:
+            line = line.strip()
+            if not line or line.startswith("#"):
+                continue
+            key, _, value = line.partition("=")
+            key = key.strip()
+            value = value.strip().strip('"').strip("'")
+            if not key or not value:
+                continue
+            # Escape for C string literal
+            escaped = value.replace("\\", "\\\\").replace('"', '\\"')
+            env.Append(CPPDEFINES=[(key, env.StringifyMacro(escaped))])
+            print(f"  .env: {key} = {value[:20]}...")