Multi-device support, dynamic grid adaptation, keyboard action

- Multi-device: OpenAllDecks(), GET /api/decks, per-deck PageManagers
- Dynamic grid: frontend adapts to connected device layout
- Keyboard action: new action type for sending key combos via wtype/xdotool
- Key recorder UI: click-based modifier toggles + single key capture
- Config: remove hardcoded maxKey validation
- Screensaver and auto-switch sync across all decks

internal/config/config.go

@@ -160,14 +160,10 @@ func (c *Config) Validate() error {
 		pageNames[p.Name] = true
 
 		seenKeys := make(map[int]bool)
-		maxKey := 31
 		for _, k := range p.Keys {
 			if k.Index < 0 {
 				return fmt.Errorf("page %s: key index must be >= 0", p.Name)
 			}
-			if k.Index > maxKey {
-				return fmt.Errorf("page %s: key index %d exceeds max %d", p.Name, k.Index, maxKey)
-			}
 			if seenKeys[k.Index] {
 				return fmt.Errorf("page %s: duplicate key index %d", p.Name, k.Index)
 			}