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/settings.cpp

@@ -20,6 +20,8 @@ Settings SettingsManager::get() {
     s.img_quality   = static_cast<ImageQuality>(readU8("img_quality", DEFAULT_IMG_QUALITY));
     s.meta_flags    = readU8("meta_flags", DEFAULT_META_FLAGS);
     s.meta_pos      = static_cast<MetaPosition>(readU8("meta_pos", DEFAULT_META_POS));
+    s.date_fmt      = readString("date_fmt", "%Y-%m-%d");
+    s.time_fmt      = readString("time_fmt", "%H:%M:%S");
     s.led_brightness = readU8("led_bright", DEFAULT_LED_BRIGHTNESS);
     s.queue_cursor  = readU32("queue_cursor", 0);
     s.albums_json   = readString("albums_json", "[]");
@@ -36,6 +38,8 @@ void SettingsManager::save(const Settings& s) {
     writeU8("img_quality", static_cast<uint8_t>(s.img_quality));
     writeU8("meta_flags", s.meta_flags);
     writeU8("meta_pos", static_cast<uint8_t>(s.meta_pos));
+    writeString("date_fmt", s.date_fmt);
+    writeString("time_fmt", s.time_fmt);
     writeU8("led_bright", s.led_brightness);
     writeU32("queue_cursor", s.queue_cursor);
     writeString("albums_json", s.albums_json);