fix: stabilize silent Windows installer smoke

[numnx]

What changed

• skip the custom NSIS beta page before nsDialogs initializes during silent installs
• explicitly select current-user mode in the isolated Windows installed-app smoke
• preserve the existing bounded retry policy for the exact transient access-violation status
• document the interactive and non-interactive installer behavior

Why

The Windows release-candidate installer could terminate with 0xC0000005 on every retry before the packaged application started. The custom page had no silent-mode guard, so non-interactive release and deployment installs could enter UI plugin initialization without an interactive desktop.

Validation

• node_modules/.bin/vitest run tests/backend/electron-builder-config.test.ts — 7 passed
• node_modules/.bin/vitest run tests/backend — 6,151 passed, 12 skipped
• node_modules/.bin/tsc --noEmit --pretty false — passed
• node scripts/build.mjs — passed
• Electron Builder reached Windows packaging locally; native completion requires Wine and is validated by the Windows CI runner

Risk and rollback

Interactive installers still show the beta notice. Silent installs skip only that custom page and continue through the standard NSIS installation flow. Roll back this commit to restore the previous page behavior.