Phase 7: region box editor in the wallpaper maker

"Edit cell boxes" mode: the selected cell grows resize handles on the
preview -- drag to move/resize with live outline tracking and a re-render
on drop, or type exact X/Y/W/H in the new geometry fields. Plain clicks
still select and cycle stacked cells (click-vs-drag resolved by a movement
threshold on mouse-up). The move/resize math is pure and unit-tested
(OverlayRegionEdit: corner/edge/move handle hit-testing with tolerance,
min-size clamped resizing that pins the opposite edge). "Save template"
persists the shared cell geometry back to the regions.json; "Reload
template" re-reads it to discard unsaved box edits. Cursor feedback per
handle; template save/reload disabled when no template path was supplied.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/PLAN.md

@@ -239,11 +239,16 @@ Replaces the legacy Google-Sheet → `.data` → GIMP → Script-Fu pipeline
   exercised end-to-end on the real cockpit art (`regions.json` + `riojoy.png` +
   `TEST.data`) by `OverlayRenderIntegrationTests`. `RioJoy.Tray/WallpaperApplier`
   applies the result via `SystemParametersInfo`.
-- **Region authoring — extraction done ✅.** `tools/XcfRegionExtract` parses the
+- **Region authoring — done ✅.** `tools/XcfRegionExtract` parses the
   GIMP source (`riojoy.xcf`) and writes the 119-cell
   `docs/reference/customBackground/regions.json` (per-layer offsets/size/font/color,
   `BaseImagePath` → exported `riojoy.png`), anchored by `CockpitRegionsTests`.
-  ⏳ Still to do: an in-app box editor for tweaks.
+  The **in-app box editor** lives in the wallpaper maker ("Edit cell boxes"): the
+  selected cell grows resize handles — drag to move/resize (plain clicks still
+  cycle stacked cells), or type exact X/Y/W/H — with the move/resize math in
+  `RioJoy.Core/Overlay/OverlayRegionEdit` (unit-tested, min-size clamped).
+  "Save template" persists the shared geometry back to the regions.json;
+  "Reload template" discards unsaved box edits.
 - **Runtime wiring — done ✅ (opt-in).** `RioJoy.Overlay/ProfileWallpaperGenerator`
   renders a profile's labels onto the template base image; `RioCoordinator`
   generates + applies the wallpaper on profile activation when