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.h

@@ -16,12 +16,19 @@ enum class ImageQuality : uint8_t {
 };
 
 enum class MetaPosition : uint8_t {
-    Bottom = 0,
-    Top = 1
+    // Captioned mode (full-width bar)
+    CaptionBottom = 0,
+    CaptionTop = 1,
+    // Overlay mode (corner badge)
+    OverlayTopLeft = 2,
+    OverlayTopRight = 3,
+    OverlayBottomLeft = 4,
+    OverlayBottomRight = 5
 };
 
 // Metadata flags bitmask
 constexpr uint8_t META_DATE     = 0x01;
+constexpr uint8_t META_TIME     = 0x20;
 constexpr uint8_t META_LOCATION = 0x02;
 constexpr uint8_t META_PEOPLE   = 0x04;
 constexpr uint8_t META_ALBUM    = 0x08;
@@ -44,6 +51,8 @@ struct Settings {
     // Display
     uint8_t meta_flags;
     MetaPosition meta_pos;
+    String date_fmt;  // strftime-style, default "%Y-%m-%d"
+    String time_fmt;  // strftime-style, default "%H:%M:%S"
 
     // Device
     uint8_t led_brightness;