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.h

@@ -15,7 +15,7 @@ public:
     String current();
     size_t size();
     bool needsResync();
-    String findPortraitPair(size_t startIndex);
+    String findNextPortrait();
 
 private:
     ImmichClient* _client = nullptr;