fix(ui): default floating dialog position to bottom-left

When the dialog first appears (mount) and when it switches to 'free' mode
(setMode), pin it to the bottom-left corner of the viewport (left: 20px,
bottom: 20px) rather than the previous bottom-right default. The header
drag handler still uses left/top for the new position, so this change
flows through cleanly on subsequent drags.

Note: existing users with a saved pos.x/pos.y in localStorage will still
see the dialog at the old transform-offset position until pos is reset
in a follow-up migration (see next commit).

src/ui.js

@@ -100,7 +100,7 @@ export class Dialog {
 
     if (initialMode === 'free') {
       root.style.bottom = '20px';
-      root.style.right = '20px';
+      root.style.left = '20px';
       if (initialPos.x || initialPos.y) {
         root.style.transform = `translate(${initialPos.x}px, ${initialPos.y}px)`;
       }
@@ -124,7 +124,7 @@ export class Dialog {
     this.root.style.right = '';
     if (mode === 'free') {
       this.root.style.bottom = '20px';
-      this.root.style.right = '20px';
+      this.root.style.left = '20px';
     } else if (anchorInfo && anchorInfo.canAnchor) {
       this._positionAnchored(anchorInfo.rect);
     } else {