chore(deps): bump vitest 4, vite 8, vite-plugin-svelte 7, typescript 6 (with adaptations) (#849)

## What

Bundles the four breaking major-version dependency bumps together with
the code adaptations they require, so they can land green as one unit:

| Package | From | To |
|---|---|---|
| `vitest` | 3.2.4 | 4.1.9 |
| `vite` | 7.x | 8.0.16 |
| `@sveltejs/vite-plugin-svelte` | 6.x | 7.1.2 |
| `typescript` | 5.3.3 | 6.0.3 |
| `svelte2tsx` | 0.7.x | 0.7.56 _(TS6 support)_ |
| `svelte-check` | 4.2.x | 4.6.0 _(TS6 support)_ |

`esbuild` drops out of the lockfile — Vite 8 replaces esbuild/Rollup
with Oxc/Rolldown, so it's no longer a transitive dependency.

## Code adaptations

- **`infinitescroll.mock.ts`** — `IntersectionObserverPassive` is now a
class. Vitest 4 forbids invoking a `vi.fn()` configured via
`mockReturnValue` with `new`. Also added `scrollMargin` to the observer
mocks (now required by the TS 6 `lib.d.ts` `IntersectionObserver`
interface).
- **`ScrollSentinel.spec.ts`** — the mock observer is stubbed via
`vi.stubGlobal`; Vitest 4 cannot construct an arrow-function mock
implementation with `new`. Added `scrollMargin` to the mock.
- **`vitest.setup.ts`** — cast Node's `TextEncoder` to the DOM type; TS
6 made `Uint8Array` generic, breaking the previous direct assignment.
- **`i18n.types.mjs`** — read the JSON file as `utf-8`; TS 6 types
`readFileSync` without an encoding as a `Buffer`, not a `string`.

## `overrides`

- `@dfinity/eslint-config-oisy-wallet` → `typescript: $typescript` — the
config still declares a `typescript@^5` peer; it works fine with TS 6,
so this satisfies the peer without `--legacy-peer-deps` (which would
break auto-install of the `svelte` peer).
- `@typescript-eslint/eslint-plugin` pinned to `8.58.1` (main's version)
so the refreshed lockfile doesn't change lint behavior.

## Verification

All gates that pass on `main` still pass locally:

- ✅ `tsc -p tsconfig.spec.json` — 0 errors
- ✅ tests — **331 passed** (60 files)
- ✅ `npm run check` (svelte-check + TS6) — 0 errors
- ✅ `npm run build` (Vite 8) and `npm run package` (svelte-package)
- ✅ formatting — clean
- ⚠️ `lint` shows the **same 19 pre-existing errors as `main`**
(`vitest/no-conditional-expect`, `svelte/no-navigation-without-resolve`)
— this PR adds **zero** new lint errors. `lint` and `e2e` are already
red on `main`, independent of this change.

## Note

This does **not** close the existing Dependabot PRs (#845, #843, #824) —
they're left untouched. This PR is an alternative that bundles the bumps
with the necessary adaptations.

---------

Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

Author: AntonioVentilii

e2e/busy-screen.e2e.ts-snapshots/Should-hide-spinner-1-Google-Chrome-linux.png

@@ -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"],
+    },
+  },
 ];