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/display_manager.h

@@ -16,7 +16,13 @@ public:
     void showSetupScreen();
     void showImage(const ProcessedImage& img);
     void showMessage(const char* title, const char* body);
-    void showMetadata(const AssetInfo& info, uint8_t metaFlags, MetaPosition pos);
+    void showMetadata(const AssetInfo& info, uint8_t metaFlags, MetaPosition pos,
+                      const String& dateFmt = "%Y-%m-%d", const String& timeFmt = "%H:%M:%S");
+    void showMetadataInRegion(const AssetInfo& info, uint8_t metaFlags, MetaPosition pos,
+                              uint16_t regionX, uint16_t regionY, uint16_t regionW, uint16_t regionH,
+                              const String& dateFmt = "%Y-%m-%d", const String& timeFmt = "%H:%M:%S");
+    void showSleepIndicator(MetaPosition metaPos);
+    void refresh();
 
 private:
     PowerManager* _power = nullptr;