feat(#994): tabbar overflow menu — settings and logs in ⋮ by default

Add a ⋮ overflow button at the right end of the dashboard tab strip.

SETTINGS and LOGS always live in the overflow dropdown (marked
data-overflow="default" in index.html — never rendered in the main bar).

Dynamic overflow: when the bar is too narrow to fit all remaining tabs,
rightmost tabs spill into the dropdown right-to-left. Implemented via
JS measurement + ResizeObserver; no CSS-only relayout trick needed.

The ⋮ button:
- hidden when the dropdown is empty (wide screens with only the default
  tabs overflowed, which are always there anyway → button always shown)
- gets .tabbar-overflow-active when the current hash tab is inside
- dropdown items clone the tab label + count badge from the original
- closes on outside click / Escape

MutationObserver on the tabbar catches P33RS/M4TR1X hidden-attribute
changes so the overflow recalculates when those tabs are shown/hidden.

frontend/packages/dashboard/src/dashboard.css

@@ -45,6 +45,7 @@ body.dashboard-shell {
 
 .tabbar {
   display: flex;
+  flex-wrap: nowrap;
   align-items: center;
   gap: 0.2em;
   padding: 0.5em 1em 0;
@@ -98,6 +99,104 @@ body.dashboard-shell {
   color: var(--red);
 }
 
+/* Overflowed tabs are hidden from the bar (moved to the ⋮ dropdown). */
+.tabbar .tab[data-overflow="default"],
+.tabbar .tab.tab-overflowed {
+  display: none;
+}
+
+/* ── overflow ⋮ button + dropdown ────────────────────────── */
+.tabbar-overflow {
+  position: relative;
+  flex-shrink: 0;
+  margin-left: auto; /* push to the far right */
+  align-self: stretch;
+  display: flex;
+  align-items: center;
+}
+
+.tabbar-overflow-btn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: none;
+  border: 1px solid transparent;
+  border-radius: 4px 4px 0 0;
+  color: var(--muted);
+  font-size: 1.2em;
+  line-height: 1;
+  padding: 0.3em 0.5em;
+  cursor: pointer;
+  transition: color 0.15s, background 0.15s;
+  white-space: nowrap;
+}
+.tabbar-overflow-btn:hover {
+  color: var(--purple);
+  background: rgba(203, 166, 247, 0.06);
+}
+.tabbar-overflow-btn.tabbar-overflow-active {
+  color: var(--purple);
+  border-color: var(--purple-dim);
+}
+
+.tabbar-overflow-dropdown {
+  position: absolute;
+  right: 0;
+  top: calc(100% + 2px);
+  z-index: 100;
+  background: var(--surface0);
+  border: 1px solid var(--surface2);
+  border-radius: 6px;
+  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.4);
+  list-style: none;
+  margin: 0;
+  padding: 0.3em 0;
+  min-width: 12em;
+  white-space: nowrap;
+}
+
+.tabbar-overflow-item {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 0.6em;
+  width: 100%;
+  background: none;
+  border: none;
+  color: var(--text);
+  font-family: inherit;
+  font-size: 0.88em;
+  letter-spacing: 0.05em;
+  text-align: left;
+  text-decoration: none;
+  padding: 0.45em 1em;
+  cursor: pointer;
+}
+.tabbar-overflow-item:hover {
+  background: var(--surface1);
+  color: var(--purple);
+}
+.tabbar-overflow-item-active {
+  color: var(--purple);
+}
+
+.tabbar-overflow-badge {
+  display: inline-block;
+  background: var(--purple-dim);
+  color: var(--purple);
+  border-radius: 999px;
+  padding: 0 0.5em;
+  min-width: 1.4em;
+  text-align: center;
+  font-size: 0.82em;
+  font-variant-numeric: tabular-nums;
+  font-weight: bold;
+}
+.tabbar-overflow-badge.tab-count-attn {
+  background: rgba(243, 139, 168, 0.22);
+  color: var(--red);
+}
+
 /* Tab pane visibility — show only the active one. The .tab-pane-active
    class is set by tabs.js based on the URL hash; default (no hash)
    resolves to SW4RM. */