refactor(frontend): migrate dashboard tabbar to createTabStrip + drop overflow (#1464 step 1)

The dashboard's tab strip now runs on the shared createTabStrip
(@hive/shared/tabs.js) — the third and final consumer of #1464 step 1
(after logs + the agent window selector).

- activateTab is reduced to the per-tab side-effects only
  (body.dataset.activeTab, the selection-bar gate, the lazy-loads);
  createTabStrip owns the active tab/pane toggle + aria-selected + hash
  routing, calling activateTab via onShow. Panes switch via the `hidden`
  attribute (was a `.tab-pane-active` class) — the markup carries `hidden`
  so there's no flash before the script runs.
- the responsive overflow menu is DROPPED per the design call (move panes
  to their own pages rather than hide them in a dropdown). Removes ~150
  lines: updateTabbarOverflow, closeOverflowMenu, the click/keydown
  handlers, the ResizeObserver + MutationObserver, and syncTabFromHash.
  S3TT1NGS is a regular strip tab now (no data-overflow).
- CSS: .tab.active becomes .tab.hive-tab--active; pane visibility is now
  .tab-pane[hidden] { display: none }; the overflow dropdown styles are
  gone. Count pills (setTabCount/refreshTabCounts) are unchanged.

Behaviour-preserving except the deliberate overflow removal; aria-selected
is now standardised on the tabs.

frontend/packages/dashboard/src/dashboard.css

@@ -85,7 +85,7 @@ body.dashboard-shell {
   color: var(--purple);
   background: color-mix(in srgb, var(--purple) 6%, transparent);
 }
-.tabbar .tab.active {
+.tabbar .tab.hive-tab--active {
   color: var(--purple);
   border-color: var(--purple-dim);
   background: var(--bg);
@@ -112,109 +112,11 @@ 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: color-mix(in srgb, var(--purple) 6%, transparent);
-}
-.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(--bg-elev);
-  border: 1px solid var(--purple-dim);
-  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(--fg);
-  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(--border);
-  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: color-mix(in srgb, var(--red) 22%, transparent);
-  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. */
-.tab-pane { display: none; }
-.tab-pane.tab-pane-active { display: block; }
+/* Tab pane visibility — createTabStrip (@hive/shared/tabs.js) sets the
+   `hidden` attribute on every inactive pane (resolved from the URL hash;
+   default — no hash — is SW4RM). Only the active pane stays visible. */
+.tab-pane[hidden] { display: none; }
 
 /* FL0W is a separate page (`/flow.html`) — its full-viewport
    styling lives in flow.css. */