feat: thinner border (4 px) and serial logging on border / recovery events

- BORDER_THICKNESS_PX: 16 -> 4. Hardware-tested at 4 px on both yellow
  and red; yellow appears slightly thicker due to the irradiation
  illusion (perception, not a rendering issue) — not compensating per
  color absent an explicit request.
- Add Serial.println at every state transition that touches the
  err_border lifecycle: schema migration firing, sync-fail else
  branch (with HTTP code, distinguishing border vs full-fill fallback),
  304 recovery (with which flags triggered it), and recovery completion
  / abort. Lets us trace why a frame is or isn't showing a border via
  pio device monitor without needing to instrument anew each time.

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

src/config.h

@@ -38,7 +38,7 @@
 #define NVS_SCHEMA_VERSION  1
 
 // Width of the sync-fail / no-WiFi border, in pixels.
-#define BORDER_THICKNESS_PX 16
+#define BORDER_THICKNESS_PX 4
 
 // ── Network ──────────────────────────────────────────────────────────────────
 #define APP_BASE_URL      "https://pictureframe.edholm.me"

src/operation.h

@@ -77,6 +77,8 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
     // by treating this boot as if errBorder were set, then bump schema_v so
     // it doesn't fire again.
     if (schemaV < NVS_SCHEMA_VERSION) {
+        Serial.println(String("[op] schema migration v") + String(schemaV) + " -> v" +
+                       String(NVS_SCHEMA_VERSION) + ", forcing one-shot recovery redraw");
         errBorder = true;
         prefs.begin(NVS_NAMESPACE, false);
         prefs.putInt(NVS_KEY_SCHEMA_V, NVS_SCHEMA_VERSION);
@@ -135,6 +137,9 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
         // (drawNeeded), or a sync-fail border is currently on screen and the
         // server is healthy again (errBorder) — repaint clean to clear it.
         if (drawNeeded || errBorder) {
+            Serial.println(String("[op] 304 with recovery flags (drawNeeded=") +
+                           String((int)drawNeeded) + " errBorder=" +
+                           String((int)errBorder) + ") -> repainting clean from /img.bin");
             File r = LittleFS.open(IMAGE_PATH, "r");
             if (r) {
                 displayInitialized = true;
@@ -145,6 +150,9 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
                 prefs.putInt(NVS_KEY_DRAW_NEEDED, 0);
                 prefs.putInt(NVS_KEY_ERR_BORDER, 0);
                 prefs.end();
+                Serial.println("[op] recovery redraw complete; flags cleared");
+            } else {
+                Serial.println("[op] recovery aborted: /img.bin not in LittleFS");
             }
         }
     } else if (code == 204) {
@@ -167,9 +175,13 @@ void normal_operation_impl(const String& mac, HTTP& http, const String& url, Pre
         epd_init();
         File r = LittleFS.open(IMAGE_PATH, "r");
         if (r) {
+            Serial.println(String("[op] sync fail code=") + String(code) +
+                           " -> drawing image with yellow border");
             epd_draw_image_with_border(r, COLOR_YELLOW, BORDER_THICKNESS_PX);
             r.close();
         } else {
+            Serial.println(String("[op] sync fail code=") + String(code) +
+                           " -> no cached image, falling back to full yellow fill");
             epd_fill(COLOR_YELLOW);
         }
         prefs.begin(NVS_NAMESPACE, false);