feat(plugin): ModuleEvent::PluginFileDialog for native file import

Add-ons had no way to request a file picker — only the host's own Open
dialog. PluginFileDialog lets a plugin tool ask the host to open a native
picker; on selection the host dispatches "<command> <path>" back to the
plugin with original case preserved (bypassing the command-line
upper-casing that mangles case-sensitive paths on Linux/macOS). The demo
plugin gains an Import tool exercising it.

Part of the #100 extensibility epic (surfaced in #106).

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

src/ui/ribbon/widgets.rs

@@ -1269,6 +1269,12 @@ pub fn module_event_to_message(event: ModuleEvent) -> Message {
         ModuleEvent::ClearModels => Message::ClearScene,
         ModuleEvent::SetWireframe(w) => Message::SetWireframe(w),
         ModuleEvent::ToggleLayers => Message::ToggleLayers,
+        // Needs the tool context + async picker — route through the normal
+        // ribbon-click handler rather than a direct 1:1 message.
+        e @ ModuleEvent::PluginFileDialog { .. } => Message::RibbonToolClick {
+            tool_id: String::new(),
+            event: e,
+        },
     }
 }