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/stimulus.ts

@@ -0,0 +1,3 @@
+import { startStimulusApp } from '@symfony/stimulus-bundle';
+
+startStimulusApp();