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>

test/mocks/HTTPClient.h

@@ -0,0 +1,47 @@
+#pragma once
+#include "Arduino.h"
+#include "LittleFS.h"
+#include "WiFiClientSecure.h"
+#include <map>
+
+// Global test state for inspecting behavior
+extern int   g_http_get_code;
+extern std::map<std::string, std::string> g_http_response_headers;
+extern std::map<std::string, std::string> g_http_request_headers;
+extern bool  g_http_end_called;
+extern std::string g_http_body;
+
+struct MockHTTPClient {
+    bool _ended = false;
+
+    void begin(WiFiClientSecure&, const String& url) {}
+
+    void addHeader(const char* name, const String& value) {
+        g_http_request_headers[name] = value._s;
+    }
+
+    void collectHeaders(const char** headers, int count) {}
+
+    int GET() {
+        _ended = false;
+        return g_http_get_code;
+    }
+
+    String header(const char* name) {
+        if (_ended) return String("");
+        auto it = g_http_response_headers.find(name);
+        return it != g_http_response_headers.end() ? String(it->second) : String("");
+    }
+
+    size_t writeToStream(File* f) {
+        if (f && *f) {
+            f->write((const uint8_t*)g_http_body.data(), g_http_body.size());
+        }
+        return g_http_body.size();
+    }
+
+    void end() {
+        _ended = true;
+        g_http_end_called = true;
+    }
+};