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/config.h

@@ -0,0 +1,28 @@
+#pragma once
+// Mirror of src/config.h for use in native unit tests.
+// Values must match src/config.h so test assertions stay consistent.
+#define APP_BASE_URL      "https://pictureframe.edholm.me"
+#define NVS_NAMESPACE     "pf"
+#define NVS_KEY_SSID        "ssid"
+#define NVS_KEY_PASS        "pass"
+#define NVS_KEY_IMG_ID      "img_id"
+#define NVS_KEY_DRAW_NEEDED "draw"
+#define IMAGE_PATH        "/img.bin"
+#define FETCH_INTERVAL_MS 60000ULL
+#define WIFI_TIMEOUT_MS   30000
+#define RESET_HOLD_MS     5000
+#define AP_IP             "192.168.4.1"
+#define PIN_CS   5
+#define PIN_DC   17
+#define PIN_RST  16
+#define PIN_BUSY 4
+#define PIN_SCK  18
+#define PIN_MOSI 23
+#define PIN_BTN_RESET 0
+// Color constants (also defined in Arduino mock, repeated here for clarity)
+#define COLOR_BLACK   0x0
+#define COLOR_WHITE   0x1
+#define COLOR_YELLOW  0x2
+#define COLOR_RED     0x3
+#define COLOR_BLUE    0x5
+#define COLOR_GREEN   0x6