busbars: FDM test-print export (extruded STL + STEP)

Adds a way to print a busbar on a 3D printer to physically verify
sizing before laser-cutting nickel/copper. The existing "Extrude
solid + 0.2mm" path stays for thin-strip STEP — slicers can't print
0.2mm sheet — so the FDM path has its own thickness input (default
2mm, min 0.5mm).

Backend:
- busbar_export.py: new to_stl() writer. Forces extrude_flag = True
  (STL is inherently 3D) and bumps thickness up to 2mm if the
  incoming value is <0.5mm. Registers under WRITERS["stl"] so the
  existing /api/export/<fmt> route serves it for free.

UI:
- index.html: new "FDM test print" block under the Params panel
  with its own thickness input and two buttons (STL, STEP). The
  existing 'Extrude solid' checkbox + 0.2mm thickness keep
  driving plain "Export STEP".
- styles.css: .fdm-block / .fdm-row / .fdm-buttons styles
  matching the existing panel typography.
- app.js: _exportFdm(fmt) reuses Exporter.exportFormat with a
  shallow-merged params override ({extrude: true, thickness: fdmT}),
  so the on-the-fly request gets the FDM settings without
  mutating the live params state.

Verified: STL render of a 3-cell strip @ 2mm => 73KB binary STL
(opens cleanly in slicers); STEP @ 2mm => 160KB ISO-10303-21
solid; existing flat STEP path unchanged at 15KB.

busbar_export.py

@@ -46,6 +46,7 @@ from build123d import (
     Sketch,
     add,
     export_step,
+    export_stl,
     extrude,
 )
 
@@ -299,8 +300,29 @@ def to_svg(payload: dict) -> bytes:
         return path.read_bytes()
 
 
+def to_stl(payload: dict) -> bytes:
+    """STL with the busbars extruded to a printable plate.
+
+    STL is inherently 3D, so extrusion is forced regardless of the payload
+    flag. Thickness defaults to 2 mm when missing or <0.5 mm (slicers can't
+    do 0.2 mm sheet — that figure is for the 'thin nickel strip' STEP case).
+    """
+    busbars, extrude_flag, thickness = parse_payload(payload)
+    if thickness < 0.5:
+        thickness = 2.0
+    shapes = build_shapes(busbars, extrude_flag=True, thickness=thickness)
+    compound = _as_compound(shapes)
+    with TemporaryDirectory() as tmp:
+        path = Path(tmp) / "busbars.stl"
+        # build123d's export_stl takes a single shape; the Compound carries
+        # all busbars together so the slicer sees them as one job.
+        export_stl(compound, str(path))
+        return path.read_bytes()
+
+
 WRITERS = {
     "step": (to_step, "application/step", "step"),
     "dxf":  (to_dxf,  "image/vnd.dxf",     "dxf"),
     "svg":  (to_svg,  "image/svg+xml",     "svg"),
+    "stl":  (to_stl,  "model/stl",         "stl"),
 }