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.

static/styles.css

@@ -420,6 +420,36 @@ label.checkbox {
   color: var(--text);
 }
 
+/* FDM test-print block inside the Params panel */
+.fdm-block {
+  margin-top: 10px;
+  padding-top: 10px;
+  border-top: 1px solid var(--border);
+}
+.fdm-title {
+  font-size: 11px;
+  font-weight: 600;
+  color: var(--muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  margin: 0 0 6px;
+}
+.fdm-row {
+  display: flex;
+  align-items: end;
+  gap: 10px;
+}
+.fdm-row label {
+  flex: 1;
+}
+.fdm-buttons {
+  display: flex;
+  gap: 4px;
+}
+.fdm-buttons button {
+  min-width: 50px;
+}
+
 input[type=number], input[type=text], select, textarea {
   background: var(--bg);
   color: var(--text);