feat v1.18.0: live segment ruler while drawing rooms + per-space scale

- Draw mode shows the current segment length near the cursor (metres or feet+inches
  per the HA unit system). New pure helpers segmentCm/formatLength (unit-tested).
- New per-space 'Scale (grid cell size)' field (space.cell_cm, default 5 cm) so each
  plan has its own real-world dimensions. i18n en/ru, .measurelabel style. +3 tests (61).

src/i18n/en.json

@@ -152,6 +152,8 @@
   "import.progress": "Floor {i} of {n}",
   "import.done": "Spaces created. Outline the rooms: click grid dots and close the contour.",
   "btn.skip": "Skip",
+  "space.scale_label": "Scale (grid cell size)",
+  "space.scale_unit": "cm per cell",
   "space.display_section": "Display",
   "space.show_borders": "Always show room borders",
   "space.show_names": "Show room names (drag to move)",