refactor: replace runtime isElectron() with build-time isDesktop constant (#8710)

## Summary

Replace all runtime `isElectron()` function calls with the build-time
`isDesktop` constant from `@/platform/distribution/types`, enabling
dead-code elimination in non-desktop builds.

## Changes

- **What**: Migrate 30 files from runtime `isElectron()` detection
(checking `window.electronAPI`) to the compile-time `isDesktop` constant
(driven by `__DISTRIBUTION__` Vite define). Remove `isElectron` from
`envUtil.ts`. Update `isNativeWindow()` to use `isDesktop`. Guard
`electronAPI()` calls behind `isDesktop` checks in stores. Update 7 test
files to use `vi.hoisted` + getter mock pattern for per-test `isDesktop`
toggling. Add `DISTRIBUTION=desktop` to `dev:electron` script.

## Review Focus

- The `electronDownloadStore.ts` now guards the top-level
`electronAPI()` call behind `isDesktop` to prevent crashes on
non-desktop builds.
- Test mocking pattern uses `vi.hoisted` with a getter to allow per-test
toggling of the `isDesktop` value.
- Pre-existing issues not addressed: `as ElectronAPI` cast in
`envUtil.ts`, `:class="[]"` in `BaseViewTemplate.vue`,
`@ts-expect-error` in `ModelLibrarySidebarTab.vue`.
- This subsumes PR #8627 and renders PR #6122 and PR #7374 obsolete.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8710-refactor-replace-runtime-isElectron-with-build-time-isDesktop-constant-3006d73d365081c08037f0e61c2f6c77)
by [Unito](https://www.unito.io)

src/components/TopMenuSection.test.ts

@@ -18,9 +18,8 @@ import { useCommandStore } from '@/stores/commandStore'
 import { useExecutionStore } from '@/stores/executionStore'
 import { TaskItemImpl, useQueueStore } from '@/stores/queueStore'
 import { useSidebarTabStore } from '@/stores/workspace/sidebarTabStore'
-import { isElectron } from '@/utils/envUtil'
 
-const mockData = vi.hoisted(() => ({ isLoggedIn: false }))
+const mockData = vi.hoisted(() => ({ isLoggedIn: false, isDesktop: false }))
 
 vi.mock('@/composables/auth/useCurrentUser', () => ({
   useCurrentUser: () => {
@@ -30,7 +29,13 @@ vi.mock('@/composables/auth/useCurrentUser', () => ({
   }
 }))
 
-vi.mock('@/utils/envUtil')
+vi.mock('@/platform/distribution/types', () => ({
+  isCloud: false,
+  isNightly: false,
+  get isDesktop() {
+    return mockData.isDesktop
+  }
+}))
 vi.mock('@/stores/firebaseAuthStore', () => ({
   useFirebaseAuthStore: vi.fn(() => ({
     currentUser: null,
@@ -107,6 +112,8 @@ describe('TopMenuSection', () => {
   beforeEach(() => {
     vi.resetAllMocks()
     localStorage.clear()
+    mockData.isDesktop = false
+    mockData.isLoggedIn = false
   })
 
   describe('authentication state', () => {
@@ -129,7 +136,7 @@ describe('TopMenuSection', () => {
 
       describe('on desktop platform', () => {
         it('should display LoginButton and not display CurrentUserButton', () => {
-          vi.mocked(isElectron).mockReturnValue(true)
+          mockData.isDesktop = true
           const wrapper = createWrapper()
           expect(wrapper.findComponent(LoginButton).exists()).toBe(true)
           expect(wrapper.findComponent(CurrentUserButton).exists()).toBe(false)