Convert frontend code to typescript (#31559)

None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
https://github.com/go-gitea/gitea/pull/31559/commits/61105d0618e285d97e95044bfb64415f364a4526
because of a deprecation that was not showing before the rename).

`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.

Everything appears to work fine.

tsconfig.json

@@ -26,5 +26,10 @@
     "noUnusedParameters": true,
     "noPropertyAccessFromIndexSignature": false,
     "exactOptionalPropertyTypes": false,
+    "sourceMap": true,
+    "types": [
+      "vitest/globals",
+      "./types.d.ts",
+    ],
   }
 }