Add conflict policy options: Skip / Auto-number / Override

New ConflictPolicy enum in rename/ (Skip = original/default behavior,
AutoNumber = save as "Name (1).ext" counting up to the first free slot,
Override = replace the conflicting file outright), threaded through
execute_renames/rename_one alongside RenameAction. Override never deletes
a conflicting directory (falls back to Skip for that case); AutoNumber
sidesteps directories the same way by just trying the next number.
execute_rename now takes a conflict argument; undo/history are
deliberately NOT conflict-aware and always use their own fixed
skip-on-conflict policy regardless of what the original rename used.

Frontend: a second segmented selector in the rename confirmation modal
("On conflict: Skip / Auto-number / Override"), with Override styled in
red and its warning copy calling out that it can't be undone.

src/components/RenamePanel.css

@@ -497,6 +497,19 @@
   pointer-events: none;
 }
 
+.action-segmented-option-danger.selected {
+  background: #ff8a8a;
+  color: var(--dock-deep);
+}
+
+.action-segmented-option-danger:not(.selected):hover {
+  color: #ff8a8a;
+}
+
+.modal-note-danger {
+  color: #ff8a8a;
+}
+
 .modal-field-label {
   display: block;
   font-size: 11px;