Refactor the TypeScript data flow for full type safety and auto-generation of Rust types (#3865)

* Migrate Specta to Tsify to auto-generate messages.ts, working except colors and widgets

* Adopt the generated FillColor/Color/GradientStops

* Fix widget typing

* Separate WidgetGroup enum variants into wrapper structs

* Small rename

* Simplify widgets further

* Clean up message type references

* Switch type imports to the auto-generated file

* Remove lowercase serde rename

* Fix FillChoice deserialization

* Fix small regression from #3837

* Improve type safety

* Make WidgetSpan type-safe

* More cleanup and type safety

* More type safety

* More type safety

* Get the rest to type-check without errors; improve widget builder macro to have optional icons; improve Svelte 5 configs

* Cargo fmt

* Fix imports

* Update outdated readme info

* Fix lint command rename references

* Fix typos

* One more typos fix

* Remove unnecessary dep: prefix from the edited Cargo.toml files

* Remove excess parts from Cargo.toml

* Fix compiling on desktop

* Revert "Remove excess parts from Cargo.toml"

This reverts commit 6b711117b3a5d5d8a3ee20f36a43bc74930b7c82.

* Update dev docs with simpler, more accurate instructions

desktop/wrapper/src/intercept_frontend_message.rs

@@ -26,6 +26,7 @@ pub(super) fn intercept_frontend_message(dispatcher: &mut DesktopWrapperMessageD
 			});
 		}
 		FrontendMessage::TriggerSaveDocument { document_id, name, path, content } => {
+			let content = content.into_vec();
 			if let Some(path) = path {
 				dispatcher.respond(DesktopFrontendMessage::WriteFile { path, content });
 			} else {
@@ -42,6 +43,7 @@ pub(super) fn intercept_frontend_message(dispatcher: &mut DesktopWrapperMessageD
 			}
 		}
 		FrontendMessage::TriggerSaveFile { name, content } => {
+			let content = content.into_vec();
 			dispatcher.respond(DesktopFrontendMessage::SaveFileDialog {
 				title: "Save File".to_string(),
 				default_filename: name,