chore(deps): bump vite 8, vite-plugin-svelte 7, typescript 6

Rebased onto main (#861 already bumped everything else to latest, incl.
vitest 4). This carries only the remaining hard majors that #861 deliberately
held back, plus the adaptations needed to make them install and type-check:

- package.json: vite ^8.0.16, @sveltejs/vite-plugin-svelte ^7.1.2,
  typescript ^6.0.3. SvelteKit 2.66's vite-plugin-svelte peer already allows
  ^7. TS 6 collides with @dfinity/eslint-config-oisy-wallet's `typescript@^5`
  peer, resolved via an `overrides` entry pinning the config's typescript to
  the root `$typescript`.
- IntersectionObserver mocks: add `scrollMargin` (TS 6's DOM lib added it to
  the interface).
- vitest.setup.ts: cast the global target for the TextEncoder polyfill
  (TS 6 made `Uint8Array` generic).
- eslint: disable `no-useless-assignment` for .svelte files (false positives
  on reactive `$:` reassignments).
- i18n.types.mjs: read en.json as utf-8.

Lockfile regenerated on linux/amd64; npm ci, lint, check, test (331), and
build all validated green in a CI-matching container.

Author: AntonioVentilii

eslint.config.mjs

@@ -28,4 +28,12 @@ export default [
       "vitest/expect-expect": ["off"],
     },
   },
+  {
+    files: ["**/*.svelte"],
+    rules: {
+      // Svelte reactive declarations (`$:`) reassign `let` bindings, which
+      // eslint's static analysis misreads as a useless initial assignment.
+      "no-useless-assignment": ["off"],
+    },
+  },
 ];