feat: blurred letterbox, portrait pairs, overlay metadata, button fixes

- Letterbox: extend edge pixels outward with blur + fade to solid color
- Portrait: detect and pair portraits side-by-side, fallback to solo
- Metadata: add overlay mode (corner badge), time field, custom date/time
  format strings (strftime), font size 1.5x, region-aware for pairs
- Buttons: swap G9/G10 to match physical layout, double-press random
  button for deep sleep, SLP indicator drawn before sleep
- Boot: remove redundant 'Loading photos' screen (saves 17s refresh)

Co-authored-by: Cursor <cursoragent@cursor.com>

src/photo_queue.cpp

@@ -141,10 +141,21 @@ bool PhotoQueue::needsResync() {
     return elapsed >= (QUEUE_RESYNC_HOURS * 3600000UL);
 }
 
-String PhotoQueue::findPortraitPair(size_t startIndex) {
-    for (size_t i = 1; i <= PORTRAIT_LOOKAHEAD && (startIndex + i) < _queue.size(); i++) {
-        return _queue[startIndex + i];
+String PhotoQueue::findNextPortrait() {
+    if (_client == nullptr) return "";
+
+    size_t scanEnd = min(_cursor + PORTRAIT_LOOKAHEAD, _queue.size());
+    for (size_t i = _cursor; i < scanEnd; i++) {
+        AssetInfo info = _client->fetchAssetInfo(_queue[i]);
+        if (info.isPortrait) {
+            String id = _queue[i];
+            _queue.erase(_queue.begin() + i);
+            Serial.printf("[queue] Found portrait pair: %s (scanned %d ahead)\n",
+                          id.c_str(), (int)(i - _cursor + 1));
+            return id;
+        }
     }
+    Serial.println("[queue] No portrait pair found in lookahead");
     return "";
 }