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/distribution/types.ts

@@ -1,5 +1,3 @@
-import { isElectron } from '@/utils/envUtil'
-
 /**
  * Distribution types and compile-time constants for managing
  * multi-distribution builds (Desktop, Localhost, Cloud)
@@ -15,10 +13,8 @@ declare global {
 /** Current distribution - replaced at compile time */
 const DISTRIBUTION: Distribution = __DISTRIBUTION__
 
-/** Distribution type checks */
-export const isDesktop = DISTRIBUTION === 'desktop' || isElectron() // TODO: replace with build var
+export const isDesktop = DISTRIBUTION === 'desktop'
 export const isCloud = DISTRIBUTION === 'cloud'
-// export const isLocalhost = DISTRIBUTION === 'localhost' || (!isDesktop && !isCloud)
 
 /**
  * Whether this is a nightly build (from main branch).