refactor: remove cached AppConfig pointer and read configuration directly from DB

Replace in-memory atomic AppConfig caching with direct DB queries via db.GetConfig
across application and page layers. This simplifies state management and ensures
all components always operate on the latest persisted configuration.

Key changes:
- Remove Application.Config atomic cache and refreshCache mechanism
- Eliminate PageCtx.CfgPtr and pass DB handle instead
- Replace all cfg reads with db.GetConfig(app.DB)
- Update mixer, audio, UI pages, and initialization paths accordingly
- Explicitly set AppConfig table name to avoid GORM pluralization issues
- Reduce DB connection pooling (SetMaxIdleConns(1))
- Standardize application font to SpaceMonoNerdFont-Regular
- Lower default audio volume (1.0 → 0.2)
- Add Unicode checkmark glyph to font map

Behavioral impact:
- Configuration is no longer cached in memory
- Every config access now hits the DB (more consistent, slightly more overhead)

internal/mixer/mixer.go

@@ -70,7 +70,7 @@ func Init(cfg *db.AppConfig, onUpdate func()) error {
 		return err
 	}
 
-	SetAllSinkVolumes(1.0)
+	SetAllSinkVolumes(0.20)
 
 	inited = true
 	return nil