fix(tabs): strip drag visual effects — preserve tab→terminal seam

The dragged tab's bottom edge sits right above the terminal area's
top edge. Adding opacity / box-shadow / cursor changes during drag
broke the chrome-tab → terminal background continuity (the dragged
tab no longer looked like part of the same surface as what's
below it).

Removed all stylistic rules from .chrome-tab.dragging. The two
remaining rules are mechanical, not cosmetic:
  - z-index: 5 so siblings' slide-aside translateX doesn't visually
    clip the dragged tab
  - transition: none so the tab tracks the cursor 1:1

The dragged tab now looks identical to its non-drag self; only its
position changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: claude

src-ui/src/components/center/CenterPanel.css

@@ -57,17 +57,22 @@
 }
 
 /* Active drag state — pointer-down a tab and slide it horizontally.
- * Disables the bg/color transition so the tab tracks the cursor 1:1
- * (translateX is applied inline via React state). z-index lifts it
- * above siblings and the active-tab "flare" pseudo-elements; subtle
- * elevation + cursor cue communicate the grab. The reorder commits
- * on pointerup; until then this is purely visual. */
+ * INTENTIONALLY zero visual effect (no opacity, no shadow, no cursor
+ * change): the dragged tab's bottom edge meets the terminal area's
+ * top edge, and any elevation / dimming there breaks the chrome-tab
+ * → terminal seam continuity. The tab just moves; its appearance
+ * stays identical to its non-drag self.
+ *
+ * Two rules below are mechanical, not stylistic:
+ *   z-index — ensures the dragged tab renders above siblings during
+ *             their slide-aside translateX, so it isn't visually
+ *             clipped by them.
+ *   transition: none — the tab tracks the cursor 1:1 (translateX is
+ *             applied inline via React state); a transition here
+ *             would lag behind the cursor. */
 .chrome-tab.dragging {
   z-index: 5;
-  cursor: grabbing;
-  opacity: 0.92;
   transition: none;
-  box-shadow: 0 4px 14px rgba(0, 0, 0, 0.28);
 }
 
 /* Chrome 'flare' pseudo-elements bridging the tab to the terminal background */