Add admin-managed page titles and subtitles

[OlivierJM]

Feature Parity: Page Titles Management

Legacy app lets admins set custom page titles and subtitles displayed across the platform; sparked-next hardcodes all page titles.

Legacy capabilities

• Admin sets main page titles (e.g., library page title, home page title)
• Admin sets page subtitles/descriptions
• Titles stored in DB with edit timestamps
• Applied dynamically across relevant pages

Acceptance criteria

• page_titles collection in DB (or extend settings)
• GET /api/settings/titles — fetch all page titles
• PUT /api/settings/titles/:page — update a page's title/subtitle (admin only)
• Admin UI: page titles management (list all pages, editable title + subtitle per page)
• Library, home, and other key pages read title/subtitle from DB instead of hardcoded strings
• Fallback to default strings if not configured

Identified via feature-parity audit against legacy Sparked (Meteor) app.