v1.50.3: sizes are not coordinates (HP-1502-01)

The ±4 bound from v1.50.2 measured view_box[2:4] and room w/h with the same
ruler as coordinates, so zero and negative sizes still passed the schema —
and viewBox='0 0 0 0' draws nothing on every client, with the static card
computing aspect-ratio: 0 / 0 on top. _EXTENT now requires strictly positive
sizes with a floor of one thousandth of the canvas (1 render unit — far
below any real room, keeps the maths finite); coordinates stay allowed to be
negative, a crop origin legitimately sits past the edge.

Defensive layer for stores that already hold a broken viewport: spaceModels
falls back to the whole canvas — both cards render from that model, so both
get the fallback — and a legacy rectangle with a negative size reads as the
same rectangle drawn from the other corner.

Also: the room settings button is the bottom row of the room card, and the
room name renders in the same spot in view and plan modes (owner's request,
committed earlier on dev).

tests_backend/test_validation.py

@@ -236,6 +236,27 @@ def test_geometry_magnitudes_are_bounded():
         {"id": "r", "name": "R", "poly": [[-0.2, 0], [1.3, 0], [1, 1]]}]}]})
 
 
+def test_sizes_are_not_coordinates():
+    """HP-1502-01: a size must be strictly positive — SVG refuses zero and
+    negative width/height, and the clients divide by these. view_box [0,0,0,0]
+    used to pass the shared validator and blank the plan on every client.
+    Coordinates may still be negative: a crop origin can sit past the edge."""
+    base = {"id": "s1", "title": "S", "view_box": [0, 0, 1, 1], "rooms": []}
+    for cfg in (
+        {**base, "view_box": [0, 0, 0, 0]},
+        {**base, "view_box": [0, 0, -1, -2]},
+        {**base, "view_box": [0, 0, 1, 0.0001]},  # below the 0.001 floor
+        {**base, "rooms": [{"id": "r", "name": "R", "x": 0.1, "y": 0.1, "w": 0, "h": 0.5}]},
+        {**base, "rooms": [{"id": "r", "name": "R", "x": 0.1, "y": 0.1, "w": 0.5, "h": -1}]},
+    ):
+        with pytest.raises(vol.Invalid):
+            v.CONFIG_SCHEMA({"spaces": [cfg]})
+    # negative COORDINATES stay legal, and a normal crop viewport passes
+    assert v.CONFIG_SCHEMA({"spaces": [{**base, "view_box": [-0.2, -0.1, 1.4, 1.2]}]})
+    assert v.CONFIG_SCHEMA({"spaces": [{**base, "rooms": [
+        {"id": "r", "name": "R", "x": -0.1, "y": -0.1, "w": 0.4, "h": 0.3}]}]})
+
+
 def test_openings_cap_enforced():
     """audit follow-up B5: MAX_OPENINGS was defined but never wired in."""
     many = [{"id": f"o{i}", "type": "door", "x": 0.1, "y": 0.1, "angle": 0, "length": 0.1}