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/sqlstore_test.go

@@ -407,7 +407,7 @@ func TestPruneLogs(t *testing.T) {
 	// LoadLogs ordering ties when rows share a created_at second).
 	present := make(map[string]bool, len(logs))
 	for _, l := range logs {
-		present[l] = true
+		present[l.Message] = true
 	}
 	if !present[fmt.Sprintf("log %d", maxLogRows+50-1)] {
 		t.Error("newest log was pruned")