feat: tab context menu pin submenu

M2 of pinned terminals (v0.17.0):
- TabBar: onPinTab prop (optional, hidden when unset for legacy callers)
- Terminal tabs get a 'Pin ▸' submenu (workspace/global) or 'Unpin' row
- UI vs Agent label distinction via isAgentTabId
- Non-terminal tabs get no pin entry (5-item legacy menu preserved)
- WorkbenchActions.pinTab threaded through split-pane LeafView
- Sidebar.onPinTab wires store.reduce(setTabPin) with cwd snapshot
- 12 new i18n keys synced across all 19 language dictionaries

src/client/split-pane.tsx

@@ -31,6 +31,12 @@ export interface WorkbenchActions {
   resizeSplit: (splitId: string, index: number, deltaFrac: number) => void
   /** Float a docked tab out as a free window (tab context menu entry). */
   floatTab: (tabId: string) => void
+  /**
+   * Pin/unpin a terminal tab (v0.17.0+). The shell snapshots the home cwd
+   * at pin time; null clears the pin. Optional: when undefined the tab
+   * context menu hides the pin entry (legacy callers).
+   */
+  pinTab?: (tabId: string, scope: 'workspace' | 'global' | null) => void
 }
 
 /** One divider: pointer-capture drag translating px deltas into fractions.
@@ -211,6 +217,7 @@ function LeafView(props: {
           else actions.moveTabBefore(payload, leaf.id, before)
         }}
         onFloatTab={actions.floatTab}
+        onPinTab={actions.pinTab}
       />
       {leaf.tabs.length > 0 ? (
         /*