Migrate frontend to TypeScript with Deno-based tooling (#206)

Compiles assets/*.ts via sensiolabs/typescript-bundle (standalone SWC
binary) and adds Deno for formatting, linting, type-checking, and
tests, keeping the project's no-Node/npm approach intact. Wires all
four into CI, the Justfile, and the pre-commit hook.

assets/types/global.d.ts

@@ -0,0 +1,11 @@
+// Ambient declarations for the parts of the AssetMapper toolchain that Deno's
+// module resolution doesn't otherwise understand: CSS/SCSS side-effect imports
+// (handled by AssetMapper at build time, not a JS module) and the Symfony
+// stimulus-bundle loader (a local vendor/ file, not an npm package).
+
+declare module '*.css';
+declare module '*.scss';
+
+declare module '@symfony/stimulus-bundle' {
+    export function startStimulusApp(): unknown;
+}