Feature Request: Add manual Dark Mode toggle in Settings

The Wire WebApp currently switches between light and dark mode only through system/browser settings. Users who prefer manual control cannot override the automatic theme selection. This creates usability issues for people who work in changing environments or prefer consistent theming independent of device settings.

Why this is a real issue:
• Many messaging apps offer manual theme switching.
• Users may work on shared devices or systems they cannot configure.
• Accessibility and contrast preferences vary, and automatic mode switching is not always desired.

Expected behavior:
A toggle in the Settings (e.g., Settings → Appearance) that allows users to switch manually between:
• Light mode
• Dark mode
• “Auto” (default system theme)

Steps to reproduce the issue:
1. Open the Wire WebApp.
2. Go to the Settings menu.
3. There is no option to manually change the theme.
4. Theme follows only system/browser settings.

Proposal / Suggested solution:
• Add a new section “Appearance” in Settings.
• Implement a 3-state switch: Light / Dark / Auto.
• Store the preference in user settings (local or account-based).
• Ensure theme switches without requiring a page reload.

Impact:
• Better accessibility and UX
• Consistent UI for users across devices
• Matches industry expectations for modern chat clients

Additional Notes:
This request is small in scope but high in user value and aligns with typical UI/UX standards in collaboration apps.

No files, tests, or entry points are named. Start by locating the Settings UI and the existing system/browser theme detection, then trace how user preferences are stored. Done means a Light/Dark/Auto control in Settings that persists the choice and switches the theme without a page reload.