fix: harden firmware NVS persistence, WDT, and 304 epd_sleep

Three bugs fixed:
- NVS img_id now written before epd_init/draw; new draw_needed flag in NVS
  survives power-loss mid-refresh so next boot re-draws from LittleFS instead
  of showing stale content
- epd_sleep() now only called when display was initialized this cycle,
  preventing a 60 s wait_busy() timeout on every 304 poll
- esp_task_wdt_reset() added to wait_busy() loop so the ~20 s 6-color
  refresh no longer triggers the task watchdog

Also extracts normal_operation into operation.h template and adds
a native PlatformIO test suite (16 tests) covering the full response matrix.

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

firmware/test/mocks/WebServer.h

@@ -0,0 +1,18 @@
+#pragma once
+#include <string>
+struct WebServer {
+    WebServer(int) {}
+    void on(const char*, void(*)()) {}
+    void on(const char*, int, void(*)()) {}
+    void onNotFound(void(*)()) {}
+    void begin() {}
+    void handleClient() {}
+    void stop() {}
+    bool hasArg(const char*) { return false; }
+    std::string arg(const char*) { return ""; }
+    void send(int, const char*, const char*) {}
+    void send_P(int, const char*, const char*) {}
+    void sendHeader(const char*, const char*) {}
+};
+#define HTTP_GET 0
+#define HTTP_POST 1