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/controllers/fullscreen_controller_test.ts

@@ -0,0 +1,75 @@
+import { assertEquals } from '@std/assert';
+
+// Stimulus's Controller base class constructor only does `this.context = context`
+// (see @hotwired/stimulus dist/stimulus.js), and none of this controller's methods
+// touch Stimulus-specific getters (element/scope/targets), so a plain `{}` context
+// is enough to construct a real instance. `document`/`sessionStorage` are stubbed
+// since Deno has no browser DOM.
+class FakeClassList {
+    classes = new Set<string>();
+    toggle(name: string, force?: boolean) {
+        const shouldAdd = force ?? !this.classes.has(name);
+        if (shouldAdd) this.classes.add(name);
+        else this.classes.delete(name);
+    }
+    contains(name: string) {
+        return this.classes.has(name);
+    }
+}
+
+const classList = new FakeClassList();
+let fullscreenElement: unknown = null;
+const sessionStore = new Map<string, string>();
+
+// deno-lint-ignore no-explicit-any
+(globalThis as any).document = {
+    documentElement: { classList },
+    get fullscreenElement() {
+        return fullscreenElement;
+    },
+    addEventListener: () => {},
+    removeEventListener: () => {},
+};
+// Deno defines a native `sessionStorage` accessor on globalThis (get/set pair), so a
+// plain assignment would just call through to it instead of replacing it — redefine
+// the property outright.
+Object.defineProperty(globalThis, 'sessionStorage', {
+    value: {
+        getItem: (key: string) => sessionStore.get(key) ?? null,
+        setItem: (key: string, value: string) => sessionStore.set(key, value),
+    },
+    configurable: true,
+});
+
+const { default: FullscreenController } = await import(
+    './fullscreen_controller.ts'
+);
+
+// deno-lint-ignore no-explicit-any
+function makeController(): any {
+    return new FullscreenController({} as never);
+}
+
+Deno.test('syncState adds is-fullscreen when a fullscreen element is set', () => {
+    fullscreenElement = { tagName: 'HTML' };
+    makeController().syncState();
+    assertEquals(classList.contains('is-fullscreen'), true);
+});
+
+Deno.test('syncState removes is-fullscreen when there is no fullscreen element', () => {
+    fullscreenElement = null;
+    makeController().syncState();
+    assertEquals(classList.contains('is-fullscreen'), false);
+});
+
+Deno.test('onFullscreenChange persists the fullscreen state and syncs classes', () => {
+    fullscreenElement = { tagName: 'HTML' };
+    makeController().onFullscreenChange();
+    assertEquals(sessionStore.get('tvdt-fullscreen'), '1');
+    assertEquals(classList.contains('is-fullscreen'), true);
+
+    fullscreenElement = null;
+    makeController().onFullscreenChange();
+    assertEquals(sessionStore.get('tvdt-fullscreen'), '0');
+    assertEquals(classList.contains('is-fullscreen'), false);
+});