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.

apps/web/e2e/billing.spec.ts

@@ -124,12 +124,12 @@ test.describe('Billing provider and Payple DOM flow', () => {
       })
     })
 
-    await page.goto('/login')
+    await page.goto('/app/login')
     await page.getByPlaceholder('user@studio.com').fill(email)
     await page.getByPlaceholder('••••••••').fill(password)
     await page.getByRole('button', { name: '로그인', exact: true }).click()
     await page.waitForURL(/\/dashboard/, { timeout: 15_000 })
-    await page.goto('/billing')
+    await page.goto('/app/billing')
 
     await expect(page.getByTestId('billing-current-tier')).toHaveText('FREE')
     await expect(page.getByTestId('billing-current-provider')).toContainText('없음')