Add admin theme and appearance customization

[OlivierJM]

Feature Parity: Theme & Appearance Customization

Legacy app lets admins customize platform colors and appearance; sparked-next uses a fixed Flowbite theme with no admin-controlled customization.

Legacy capabilities

• Admin can set primary color
• Admin can set secondary button color
• Admin can set sidebar color
• Dark mode toggle (available to all users)
• Changes applied in real-time and persisted
• Theme stored in settings collection and applied app-wide via ThemeContext

Acceptance criteria

• Admin settings UI for color customization (primary, secondary, sidebar/accent colors)
• Theme settings persisted in DB (settings collection or similar)
• Theme context/provider applies custom colors app-wide
• Dark mode toggle accessible to all users (not just via Flowbite toggle — persisted to user preference)
• GET /api/settings/theme and PUT /api/settings/theme endpoints

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