feat v1.21.0: merge and split rooms

- Merge: click a room, then a neighbour. Adjacency is decided by the RESULT, not a
  heuristic: mergeRooms unions the outlines and accepts only when they collapse into one
  hole-free outline (corner touch / apart / hole => refused). A dialog picks the surviving
  name+area; the kept room keeps its id so its label and devices stay put.
- Split: click the room, then two wall points; the chord cuts it, live ruler on the cut.
  The bigger part stays the room (name/area/devices), the smaller opens the new-room
  dialog. The cut is applied only on confirm — Cancel leaves the room whole.
- Boolean geometry via polyclip-ts (proper ESM + native types; polygon-clipping ships
  named types but a default-only ESM build, breaking either tsc or the runtime).
  Verified on the real plan, where neighbouring walls overlap collinearly rather than
  match exactly — the case a hand-rolled union gets wrong. Bundle 151->202 KB.
+5 tests (72). Verified live: merge of Сауна+с/у -> 4-vertex outline, non-adjacent
refused, split preserves area (26667 -> 13333+13333), cancel leaves the room whole.

src/styles.ts

@@ -274,6 +274,11 @@ export const cardStyles = css`
     .stage.markup .devlayer {
       display: none; /* in markup mode icons must not get in the way */
     }
+    .room.picked {
+      stroke: #ffc14d;
+      stroke-width: 3;
+      fill: rgba(255, 193, 77, 0.25);
+    }
     .room.outlined {
       stroke: rgba(62, 166, 255, 0.55);
       fill: rgba(62, 166, 255, 0.06);