chore: remove duplicate copies of the site, download center and installers (WS-C)

The landing page, download pages, invite page, assetlinks and installers
existed in two or three places; only site/ and the Forgejo feed are served.

- apps/api-server/wwwroot: delete the stale site build, download/invite
  pages, .well-known copy, legacy static admin and 1.0.0 binaries. The API
  no longer serves static files (UseStaticFiles/fallbacks and the apk/zip
  blocker removed); the Next admin is the only admin UI.
- Delete 19 tracked installers/packages (~568 MiB) under site/public/releases,
  apps/web/public/releases and wwwroot/releases; .gitignore blocks them.
- apps/web: delete the download/releases pages, desktop-release.ts, the
  download.html and assetlinks copies, and the accept-invite page (invites
  are only issued to the site's /accept-invite/). e2e specs call the /app
  base path and check the /download redirect instead.
- scripts: delete the retired release/NAS site scripts, drop the web target
  from sync-version, and check assetlinks in site/public only.
- Delete the unused Dockerfile.admin (apps/admin/Dockerfile is used).

Policy: docs/REFACTOR_POLICY.md Wave 3, W3-5 and W3-6.

tests/e2e/system-verification.spec.ts

@@ -109,11 +109,4 @@ test.describe('D3RO Voice E2E Architecture Verification', () => {
     expect(report.userSummaries.length).toBeGreaterThan(0)
     console.log('✓ BackOffice Usage & Cost Analytics verified:', report)
   })
-
-  test('6. BackOffice Web SPA Page Loading', async ({ page }) => {
-    await page.goto(`${API_BASE}/admin/index.html`)
-    await expect(page.locator('text=D3RO VOICE')).toBeVisible()
-    await expect(page.locator('text=백엔드 서버 대시보드')).toBeVisible()
-    console.log('✓ BackOffice Admin SPA UI loaded successfully.')
-  })
 })