Improve preview loop with local browser UI

[mintyPT]

Problem

clickclick preview --watch writes a PNG repeatedly, but the user still has to open or refresh the image manually. The authoring loop should feel immediate when editing HTML/CSS templates.

Proposal

Add a richer preview mode, for example:

clickclick studio ./clickclick.config.json
clickclick preview template.html --css template.css --open

This could run a small local server that shows the current render and refreshes when files change.

Acceptance criteria

• Users can open a browser-based preview from the CLI.
• Watch mode refreshes the displayed image after template or CSS changes.
• The UI shows current output path and renderer warnings.
• Supports viewport width/height options.
• Existing one-shot preview behavior remains available.
• Tests cover server startup behavior at the unit or integration boundary.