Persist selected language for plain layout pages

[AlexAndBear]

Description

This fixes language fallback behavior for pages rendered via the plain layout when no user context is available.

The runtime now resolves the initial language in this order:

1. user setting (applied when available and persisted)
2. stored language from local storage
3. browser language

This keeps the UI language stable on plain-layout pages such as /access-denied after users previously selected English.

Related Issue

• Fixes N/A

How Has This Been Tested?

• test environment: local development on macOS
• test case 1: pnpm test:unit --run packages/web-runtime/tests/unit/helpers/language.spec.ts packages/web-runtime/tests/unit/container/bootstrap.spec.ts packages/web-runtime/tests/unit/pages/account/accountPreferences.spec.ts
• test case 2: pnpm exec eslint packages/web-runtime/src/helpers/language.ts packages/web-runtime/src/container/bootstrap.ts packages/web-runtime/tests/unit/helpers/language.spec.ts
• test case 3: verified helper behavior for stored-vs-browser language fallback via unit tests

Types of changes

• Bugfix
• Enhancement (a change that does not break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)