holder: cap total cells at 1000 to prevent runaway renders

User-triggered 120x120 = 14400 cells, which produces huge STL/long
renders. Total cells is the right metric (CSG cost scales with count,
not max axis), so cap by N = rows*cols (or rows*(rows+1)/2 for tria
style). 1000 covers any realistic pack (e.g. 20x50) while blocking
accidental misuse.

Backend:
- holder.py: MAX_CELLS env-tunable (default 1000); expected_cell_count
  and _check_cell_limit raise ValueError on exceed; both
  compute_cells() and render_stl() call it up-front.
- app.py: /api/holder/render now returns 400 on ValueError (not 500)
  so the frontend can distinguish bad input from server failure.
  /api/holder/params now publishes max_cells alongside the schema.

Frontend:
- holder-app.js: reads max_cells from the params endpoint; status
  shows "N cells / over limit (1000)" in red and disables the
  Render and "Design busbars" buttons when exceeded.
- holder.css: .topbar-status.over-limit style (red, bold).

holder.py

@@ -31,6 +31,9 @@ OPENSCAD_BIN = os.environ.get("OPENSCAD_BIN", "openscad")
 # Empty string disables; default Manifold gives ~10-50x speedup on OpenSCAD 2024+
 OPENSCAD_BACKEND = os.environ.get("OPENSCAD_BACKEND", "Manifold")
 RENDER_TIMEOUT = int(os.environ.get("OPENSCAD_TIMEOUT", "300"))
+# Cap total cells to keep renders fast and STL sizes sane. 1000 covers
+# real packs (e.g. 20×50 = 1000) while blocking accidental 120×120 = 14400.
+MAX_CELLS = int(os.environ.get("HOLDER_MAX_CELLS", "1000"))
 
 
 def openscad_available() -> bool:
@@ -126,7 +129,26 @@ def schema_dict() -> list[dict]:
 _COS30 = math.cos(math.radians(30))
 
 
+def expected_cell_count(params: dict) -> int:
+    """Cell count for a pack without computing coordinates."""
+    rows  = int(params.get("num_rows", 6))
+    cols  = int(params.get("num_cols", 12))
+    style = str(params.get("pack_style", "rect"))
+    if style == "tria":
+        return rows * (rows + 1) // 2
+    return rows * cols
+
+
+def _check_cell_limit(params: dict) -> None:
+    n = expected_cell_count(params)
+    if n > MAX_CELLS:
+        raise ValueError(
+            f"Too many cells ({n} > {MAX_CELLS}). Reduce rows × cols."
+        )
+
+
 def compute_cells(params: dict) -> list[dict]:
+    _check_cell_limit(params)
     cell_dia = float(params.get("cell_dia",  21.2))
     wall     = float(params.get("wall",      0.8))
     rows     = int(params.get("num_rows", 6))
@@ -200,6 +222,7 @@ def _filter_params(params: dict) -> dict:
 
 def render_stl(params: dict) -> bytes:
     """Render the .scad with given parameter overrides; return STL bytes."""
+    _check_cell_limit(params)
     if not openscad_available():
         raise RuntimeError(
             f"`{OPENSCAD_BIN}` not found on PATH. Install OpenSCAD "