Remove the organizer's Preview and apply section

The preview/apply flow on the Organize tab did not work as expected, so
it is removed along with its backend: the stored per-pair preview, the
/organize/preview, /organize/plan and /organize/apply endpoints and
their UI and types.

The File naming card stays, and renaming now happens only through
"Organize automatically" during scans, which keeps the underlying
planning/moving code (now internal), undo, crash recovery and dry run.
The rename-folders warning is unchanged. Tests now drive the kept
planning and moving code directly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/engine.go

@@ -45,9 +45,6 @@ type Engine struct {
 	tagTools tags.Tools
 	tagCache tagCache
 
-	plansMu sync.Mutex
-	plans   map[int64]*OrganizePlan // latest organize preview per pair
-
 	jobsMu sync.Mutex
 	jobs   map[int64]*Job
 	nextID int64