fix: preserve last image and overlay yellow border on sync failure

Previously a 5xx / timeout / malformed response fired epd_fill(COLOR_YELLOW),
which writes the yellow nibble across the entire 800×480 framebuffer and
destroys the last good image — exactly what FR38 forbids ("Last image
persists ... yellow border signals state"). The device then got stuck on a
blank yellow screen because the next 304 didn't redraw.

Changes:

- New epd_draw_image_with_border streams the cached .bin row-by-row,
  overwrites border-region pixels in the row buffer, and pushes a single
  composited framebuffer (same pattern as the existing setup-QR overlay).
- normal_operation_impl else-branch now redraws the cached image with a
  yellow border, falling back to epd_fill only when no cache exists
  (first-boot error). Sets a new NVS_KEY_ERR_BORDER flag.
- 200 and 304 paths clear NVS_KEY_ERR_BORDER. The 304 branch now
  triggers a clean repaint when the err flag is set, so the device
  recovers from the stuck-yellow state on the next healthy poll
  without waiting for rotation to advance.
- LittleFS read mock now returns invalid File when the file doesn't
  exist (matches real LittleFS), so the no-cache fallback path is
  actually exercisable in tests.

Tests:

- Replaces the old test_fw06_error_fills_yellow (which locked in the
  buggy fill behavior) with FW-06a..e covering: error+cache draws
  border (no fill), error+no-cache falls back to fill, 304 after
  error repaints clean, steady-state 304 touches nothing (the
  regression the user flagged), 200 after error clears the flag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/config.h

@@ -30,6 +30,10 @@
 #define NVS_KEY_PASS        "pass"
 #define NVS_KEY_IMG_ID      "img_id"
 #define NVS_KEY_DRAW_NEEDED "draw"
+#define NVS_KEY_ERR_BORDER  "err"     // set when display is showing a sync-fail border; force a clean redraw on next 200/304
+
+// Width of the sync-fail / no-WiFi border, in pixels.
+#define BORDER_THICKNESS_PX 16
 
 // ── Network ──────────────────────────────────────────────────────────────────
 #define APP_BASE_URL      "https://pictureframe.edholm.me"