refactor: separate log timestamp from message as structured LogEntry

Introduced models.LogEntry{Message, CreatedAt} to replace raw strings
in the log pipeline. Timestamps are now formatted at render time, not
baked into stored messages.

- Engine: appendLog stores LogEntry with time.Now()
- Store: LoadLogs returns []LogEntry, selects created_at from DB
- Store: strips legacy [HH:MM] prefix from pre-refactor DB entries
- TUI: sidebar shows "MM/DD HH:MM" from CreatedAt
- TUI: full log view shows "MM/DD HH:MM" from CreatedAt
- SaveLog still receives plain message string (DB handles timestamp)

internal/store/store.go

@@ -61,7 +61,7 @@ type Store interface {
 
 	// Logs
 	SaveLog(ctx context.Context, message string) error
-	LoadLogs(ctx context.Context, limit int) ([]string, error)
+	LoadLogs(ctx context.Context, limit int) ([]models.LogEntry, error)
 	PruneLogs(ctx context.Context) error
 
 	// Maintenance Windows