Chat: use the Button primitive instead of raw <button> elements #250

Today

The app has a Button primitive (src/components/tiptap-ui-primitive/button). It gives every button the same:

• sizes and variants (ghost, …);
• hover and active states;
• dark mode;
• disabled look;
• a built-in tooltip (tooltip / showTooltip).

The chat builds its own with raw <button> and per-file CSS:

So chat buttons look and behave a bit differently from the rest of the app (focus ring, dark mode, disabled state), and each style is maintained twice.

What to do

• Replace them with Button, then delete the CSS that only re-styled the raw buttons.
• Keep a raw <button> only where Button gets in the way (for example a button laid over a message bubble). Leave a one-line comment there.
• Do it one file per PR if that's easier to review.

Done when

• No raw <button> left in src/features/chat except the commented ones.
• Chat looks the same or better, in light and dark mode.