refactor(tui): address PR #23 review, move bson/ tui/ to cmd/mxcli/

Fix all critical and moderate issues from code review:
- Fix itoa() for tab numbers >= 10 (#1)
- Add TabID to LoadTreeMsg to prevent race condition (#2)
- Fix UTF-8 truncation with runewidth (#3)
- Change View()/viewZen() to pointer receivers (#4)
- Cancel PreviewEngine on tab close and app quit (#5)
- Remove ~315 lines dead code in keys.go (#6)
- Remove unused NavState, MillerFocusPreview, GoBack, ToggleZen (#7)
- Reset traceActive in CloseTrace() (#8)
- Remove nolint:govet suppression (#9)
- Extract magic numbers as named constants (#10)
- Handle openDiagram temp file cleanup and WriteString error (#11)
- Remove redundant rebuildZones() call (#12)
- Fallback NDSL preview to MDL DESCRIBE for unsupported types
- Move bson/ and tui/ packages into cmd/mxcli/ per reviewer suggestion

Author: engalar

bson/classify.go cmd/mxcli/bson/classify.gobson/classify.go renamed to cmd/mxcli/bson/classify.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"os"
 
-	bsondebug "github.com/mendixlabs/mxcli/bson"
+	bsondebug "github.com/mendixlabs/mxcli/cmd/mxcli/bson"
 	"github.com/mendixlabs/mxcli/sdk/mpr"
 	"github.com/spf13/cobra"
 	"go.mongodb.org/mongo-driver/bson"