chore: upgrade to vite 6 (#14363)

Upgrades the frontend toolchain to Vite 6 and tidies up the build config
along the way. Behavior is unchanged for end users; this is dev/build
infra.

## What changed
- `vite` 5.4 → 6.4, `@vitejs/plugin-vue` → 5.2, `vite-plugin-ruby` → 5.2
(with matching `vite_rails`/`vite_ruby` gem bumps).
- Dropped the `vite-node` 2.0.1 pnpm override — no longer needed now
that vitest 3 runs on Vite 6 directly.
- Split the single `vite.config.ts` into:
- `vite.config.ts` (app), `vite.lib.config.ts` (SDK), `vite.shared.ts`
(aliases / Vue options), `vitest.config.ts` (tests).
- `pnpm build:sdk` now selects the SDK config explicitly instead of
branching on `BUILD_MODE=library`. SDK output path is unchanged
(`public/packs/js/sdk.js`).

No changes needed to Docker images, deployment scripts, or CI — Node 24
and pnpm 10 are already past Vite 6's floor, and the rake
`assets:precompile` hook still drives the SDK build via `pnpm`.

## How to test
- `pnpm dev` and verify the dashboard, widget, and survey routes load
and HMR works.
- Load a Chatwoot site widget on a test page and confirm `sdk.js` is
served and the widget mounts.
- `RAILS_ENV=production bundle exec rake assets:precompile` and confirm
`public/packs/js/sdk.js` plus the rest of the manifest are produced.
- `pnpm test` for the JS suite.

---------

Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>
Co-authored-by: Sony Mathew <2040199+sony-mathew@users.noreply.github.com>

app/javascript/dashboard/composables/spec/index.spec.js

@@ -10,10 +10,12 @@ vi.mock('shared/helpers/mitt', () => ({
 
 vi.mock('dashboard/helper/AnalyticsHelper/index', async importOriginal => {
   const actual = await importOriginal();
-  actual.default = {
-    track: vi.fn(),
+  return {
+    ...actual,
+    default: {
+      track: vi.fn(),
+    },
   };
-  return actual;
 });
 
 describe('useTrack', () => {

app/javascript/dashboard/composables/spec/useCaptain.spec.js

@@ -16,10 +16,12 @@ vi.mock('vue-i18n');
 vi.mock('dashboard/api/captain/tasks');
 vi.mock('dashboard/helper/AnalyticsHelper/index', async importOriginal => {
   const actual = await importOriginal();
-  actual.default = {
-    track: vi.fn(),
+  return {
+    ...actual,
+    default: {
+      track: vi.fn(),
+    },
   };
-  return actual;
 });
 vi.mock('dashboard/helper/AnalyticsHelper/events', () => ({
   CAPTAIN_EVENTS: {