Add multi-font support to countdown mode

[JanPetterMG]

This PR adds multi-font support to Countdown mode across both firmware and the web UI.

Users can now choose which supported font the countdown should use, and the selected font is exposed through the device payload, persisted in preferences, and configurable from the frontend. This also improves how the countdown behaves once it has ended.

Changes

Firmware

• Added support for multiple countdown fonts via a static list of available font names
• Persisted the selected countdown font in preferences
• Added setFont() handling for validating and applying font changes
• Included the active font and available font list in transmitted Countdown payloads
• Added Home Assistant select discovery for countdown font selection
• Updated countdown rendering to use the selected font dynamically instead of a fixed font
• Reset countdown display state in begin() using blink/lower/upper values
• Improved ended countdown handling as part of the mode update flow

Web UI

• Added Countdown font state handling in the receiver
• Added support for receiving both the active font and the list of available fonts
• Added a font selector dropdown to the Countdown sidebar
• Sent font changes back over WebSocket when a new font is selected
• Adjusted Countdown cookie initialization for hours, minutes, and seconds

Why

This makes Countdown mode more flexible and consistent with the rest of the project by allowing the display style to be selected instead of being hardcoded. It also improves the configuration experience in both the web UI and Home Assistant.

Notes

• The default countdown font is initialized from the first available compiled font
• Font changes are validated against the registered font service before being saved
• The web UI font dropdown is populated dynamically from the device-reported font list