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/platform/updates/components/ReleaseNotificationToast.test.ts

@@ -5,9 +5,7 @@ import { beforeEach, describe, expect, it, vi } from 'vitest'
 import type { ReleaseNote } from '../common/releaseService'
 import ReleaseNotificationToast from './ReleaseNotificationToast.vue'
 
-interface TestWindow extends Window {
-  electronAPI?: Record<string, unknown>
-}
+const mockData = vi.hoisted(() => ({ isDesktop: false }))
 
 const { commandExecuteMock } = vi.hoisted(() => ({
   commandExecuteMock: vi.fn()
@@ -17,6 +15,14 @@ const { toastErrorHandlerMock } = vi.hoisted(() => ({
   toastErrorHandlerMock: vi.fn()
 }))
 
+vi.mock('@/platform/distribution/types', () => ({
+  isCloud: false,
+  isNightly: false,
+  get isDesktop() {
+    return mockData.isDesktop
+  }
+}))
+
 // Mock dependencies
 vi.mock('vue-i18n', () => ({
   useI18n: vi.fn(() => ({
@@ -105,6 +111,7 @@ describe('ReleaseNotificationToast', () => {
 
   beforeEach(() => {
     vi.clearAllMocks()
+    mockData.isDesktop = false
     // Reset store state
     mockReleaseStore.recentRelease = null
     mockReleaseStore.shouldShowToast = true // Force show for testing
@@ -183,7 +190,8 @@ describe('ReleaseNotificationToast', () => {
     )
   })
 
-  it('executes desktop updater flow when running in Electron', async () => {
+  it('executes desktop updater flow when running on desktop', async () => {
+    mockData.isDesktop = true
     mockReleaseStore.recentRelease = {
       version: '1.2.3',
       content: '# Test Release'
@@ -196,7 +204,6 @@ describe('ReleaseNotificationToast', () => {
       value: mockWindowOpen,
       writable: true
     })
-    ;(window as TestWindow).electronAPI = {}
 
     wrapper = mountComponent()
     await wrapper.vm.handleUpdate()
@@ -206,11 +213,10 @@ describe('ReleaseNotificationToast', () => {
     )
     expect(mockWindowOpen).not.toHaveBeenCalled()
     expect(toastErrorHandlerMock).not.toHaveBeenCalled()
-
-    delete (window as TestWindow).electronAPI
   })
 
-  it('shows an error toast if the desktop updater flow fails in Electron', async () => {
+  it('shows an error toast if the desktop updater flow fails on desktop', async () => {
+    mockData.isDesktop = true
     mockReleaseStore.recentRelease = {
       version: '1.2.3',
       content: '# Test Release'
@@ -224,15 +230,12 @@ describe('ReleaseNotificationToast', () => {
       value: mockWindowOpen,
       writable: true
     })
-    ;(window as TestWindow).electronAPI = {}
 
     wrapper = mountComponent()
     await wrapper.vm.handleUpdate()
 
     expect(toastErrorHandlerMock).toHaveBeenCalledWith(error)
     expect(mockWindowOpen).not.toHaveBeenCalled()
-
-    delete (window as TestWindow).electronAPI
   })
 
   it('calls handleShowChangelog when learn more link is clicked', async () => {