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/settings/composables/useSettingUI.ts

@@ -6,11 +6,11 @@ import { useCurrentUser } from '@/composables/auth/useCurrentUser'
 import { useBillingContext } from '@/composables/billing/useBillingContext'
 import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import { useVueFeatureFlags } from '@/composables/useVueFeatureFlags'
-import { isCloud } from '@/platform/distribution/types'
+import { isCloud, isDesktop } from '@/platform/distribution/types'
 import type { SettingTreeNode } from '@/platform/settings/settingStore'
 import { useSettingStore } from '@/platform/settings/settingStore'
 import type { SettingParams } from '@/platform/settings/types'
-import { isElectron } from '@/utils/envUtil'
+
 import { normalizeI18nKey } from '@/utils/formatUtil'
 import { buildTree } from '@/utils/treeUtil'
 
@@ -226,7 +226,7 @@ export function useSettingUI(
       ...(shouldShowWorkspacePanel.value ? [workspacePanel] : []),
       keybindingPanel,
       extensionPanel,
-      ...(isElectron() ? [serverConfigPanel] : []),
+      ...(isDesktop ? [serverConfigPanel] : []),
       ...(shouldShowPlanCreditsPanel.value && subscriptionPanel
         ? [subscriptionPanel]
         : []),
@@ -283,7 +283,7 @@ export function useSettingUI(
         translateCategory(keybindingPanel.node),
         translateCategory(extensionPanel.node),
         translateCategory(aboutPanel.node),
-        ...(isElectron() ? [translateCategory(serverConfigPanel.node)] : [])
+        ...(isDesktop ? [translateCategory(serverConfigPanel.node)] : [])
       ]
     }),
     // Custom node settings (only shown if custom nodes have registered settings)
@@ -332,7 +332,7 @@ export function useSettingUI(
         keybindingPanel.node,
         extensionPanel.node,
         aboutPanel.node,
-        ...(isElectron() ? [serverConfigPanel.node] : [])
+        ...(isDesktop ? [serverConfigPanel.node] : [])
       ].map(translateCategory)
     }
   ])