feat: reorder & hide client tabs from Settings (also drives Overview quota cards)

[savourylie]

What

Adds user control over the top client tab bar and keeps the Overview in sync with it:

• Reorder providers by dragging them in Settings → Client tabs (top bar).
• Show / hide individual providers via a checkbox. Hidden providers stay listed in
  Settings (so you can re-enable or reorder them) but disappear from the top tabs.
• The saved order + visibility now also drive the Overview → Agent limits quota cards
  (and the Overview charts), so both views stay consistent. Reordering in Settings and
  dragging the limit cards share one persisted order.
• The active tab falls back to Overview if the user hides it.

How

• New persistence + helpers in ClientRegistry (Sources/TokenBarCore/ClientRegistry.swift):
  tabOrderKey / tabHiddenKey, hiddenClients(), displayClients(present:).
• SettingsPanel gains a "Client tabs (top bar)" section with drag-to-reorder + visibility toggles.
• PopoverView, SettingsWindowView, and AgentLimitsCard consume ClientRegistry.displayClients(...)
  so ordering/hiding applies to the tab row, the keyboard shortcuts (⌘1–9, [ / ]), and the
  Overview quota cards. The tab-row view (DashboardTabs) itself is unchanged.

Compatibility

• Preferences live in UserDefaults (tokenbar.tabs.order, tokenbar.tabs.hidden); the
  limits-card order key is unified with the tabs order key.
• No migration needed — with no saved state, behavior is unchanged (all present clients shown
  in the historical order).
• Rebased onto current main; builds clean (swift build). The diff touches 5 files and leaves
  the tab-row scroll behavior as-is.