super-buckets: split tab click (activate) from caret click (dropdown)

Clicking the super-bucket tab now activates the last-shown child
directly. Only the caret arrow on the right opens the dropdown popover.
Caret has its own hit area with accent hover highlight.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/css/style.css

@@ -821,6 +821,24 @@ body.anim-slow .board-card.card-pulse-local {
 /* Super-bucket dropdown popover. Positioned via inline styles by
    openSuperDropdown() in buckets.js. Lives on document.body (not
    inside #bucket-bar) so it can escape the bar's overflow:auto. */
+.super-caret {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    margin-left: 2px;
+    padding: 2px 4px;
+    border-radius: 3px;
+    opacity: 0.6;
+    transition: opacity 0.15s, background 0.15s;
+    cursor: pointer;
+}
+.super-caret:hover {
+    opacity: 1;
+    background: color-mix(in srgb, var(--t-accent) 25%, transparent);
+}
+.bucket-tab.active .super-caret:hover {
+    background: rgba(0,0,0,0.25);
+}
 #bucket-super-popover {
     z-index: 220;
     background: var(--t-surface);