fix(web): restore wasm build broken by SECTION + file association

The v0.7.0 web (GitHub Pages) release failed to compile under
--no-default-features. Two symbols leaked into the wasm build:

* `solid_section` (SECTION command) used truck-meshalgo directly
  (`triangulation`/`extract_interference`), but that crate is gated
  behind the `solid3d` feature which is off on wasm. Gate the function
  with `#[cfg(feature = "solid3d")]` and add a stub for builds without
  it, mirroring `solid_model::boolean`.
* `register_as_handler` was `#[cfg(not(wasm32))]` while its caller
  `set_default_app_blocking` was not, so wasm couldn't resolve it. Drop
  the wasm gate; the existing non-win/lin/mac arm already returns `Ok`
  on wasm, matching `unregister_handler`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/model_ops.rs

@@ -469,6 +469,7 @@ impl super::OpenCADStudio {
     /// SECTION — draw the cross-section outline where an axis-aligned plane
     /// (X/Y/Z = `axis` at `value`) cuts the one selected solid, as Line entities.
     /// Reuses the mesh-interference analyzer to find the cut segments.
+    #[cfg(feature = "solid3d")]
     pub(super) fn solid_section(&mut self, axis: usize, value: f64) -> Task<Message> {
         use acadrust::types::Vector3;
         use acadrust::Line;
@@ -576,6 +577,14 @@ impl super::OpenCADStudio {
         Task::none()
     }
 
+    /// Without `solid3d` (e.g. wasm) there is no mesh-interference kernel.
+    #[cfg(not(feature = "solid3d"))]
+    pub(super) fn solid_section(&mut self, _axis: usize, _value: f64) -> Task<Message> {
+        self.command_line
+            .push_error("SECTION: solid modelling is unavailable in this build.");
+        Task::none()
+    }
+
     /// PYRAMID — create an `n`-sided pyramid (regular polygon base of the given
     /// circumradius, apex at `height`) as a tessellated mesh, reusing the same
     /// face→Shell→mesh path as LOFT. Each face is independent, so no closed-shell