fix(frontend): refresh custom page document title

[Milesians]

Summary

• centralize route document title resolution for custom pages
• refresh the browser title when site settings, custom menu items, admin menu items, or locale change
• keep custom page titles using the configured menu label once settings finish loading

Verification

• ./node_modules/.bin/vitest run src/router/tests/title.spec.ts src/router/tests/wechat-route.spec.ts
• ./node_modules/.bin/vue-tsc --noEmit --pretty false
• ./node_modules/.bin/eslint src/App.vue src/i18n/index.ts src/router/index.ts src/router/title.ts src/router/tests/title.spec.ts
• git diff --check

[github-actions]

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[Milesians]

I have read the CLA Document and I hereby sign the CLA