fix(ui): scope mousedown handler to .tat-header to restore input/select focus

The dialog's mousedown listener was attached to the whole root and
unconditionally called preventDefault(), which blocked the target
element from receiving focus. As a result, the custom target <input>
and the milestone <select> could never be focused.

Only initiate drag (and only preventDefault) when the mousedown is on
the .tat-header bar. This lets the user click into inputs, selects, and
buttons inside the dialog body, while still allowing the dialog to be
dragged from the title bar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ui.js

@@ -148,6 +148,9 @@ export class Dialog {
     const self = this;
     this.root.addEventListener('mousedown', (e) => {
       if (self.mode !== 'free') return;
+      // Only initiate drag from the header bar. This prevents stealing focus
+      // from inputs, selects, and buttons inside the dialog body.
+      if (!e.target.closest('.tat-header')) return;
       if (e.target.classList.contains('tat-close')) return;
       const rect = self.root.getBoundingClientRect();
       self.dragState = { dx: e.clientX - rect.left, dy: e.clientY - rect.top };