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/i18n/en.json

@@ -27,11 +27,15 @@
   "title.configure_space": "Configure space",
   "title.add_space": "Add space",
   "title.markup_add": "Add a room: connect grid dots with lines until the outline closes",
+  "title.markup_merge": "Merge rooms: click one room, then the neighbour it shares a wall with",
+  "title.markup_split": "Split a room: click the room, then two points on its walls",
   "title.markup_delroom": "Delete a room: click inside the room",
   "title.no_area_room": "Decorative room without an HA area (e.g. a hallway)",
   "title.choose_area": "Select a Home Assistant area",
   "title.need_plan": "Upload a floor-plan image",
   "markup.add": "Add",
+  "markup.merge": "Merge",
+  "markup.split": "Split",
   "markup.delete": "Delete",
   "markup.hint_points": "points: {n} · Esc/Ctrl+Z — undo a dot · close the outline by clicking the first one",
   "markup.hint_start": "click a grid dot to start the outline",
@@ -94,6 +98,15 @@
   "toast.cfg_save_failed": "Failed to save config: {err}",
   "toast.point_in_room": "That point is inside room “{name}” — rooms must not overlap",
   "toast.room_overlap": "The outline overlaps room “{name}” — rooms must not overlap",
+  "toast.merge_not_adjacent": "Only rooms that share a wall can be merged",
+  "toast.rooms_merged": "Rooms merged into “{name}”",
+  "toast.split_pick_wall": "Click a grid dot on the room’s wall",
+  "toast.split_bad_cut": "The cut must be a straight line from wall to wall, inside the room",
+  "merge.header": "Merge rooms",
+  "merge.hint": "The merged room keeps one name and one area. The other area is released — its devices leave the plan until another room claims it.",
+  "merge.keep": "Keep",
+  "merge.no_area": "no area",
+  "room.split_header": "New room from the split",
   "toast.room_saved": "Room saved ({n}). Devices added: {added}. Outline the next one or exit markup.",
   "toast.room_saved_no_area": "Room saved ({n}, no area). Outline the next one or exit markup.",
   "toast.marker_needs_server": "Device editing is available after the config is moved to the server",