fix: free the launch window's tab manager when it closes

[arzafran]

What this does

Closing the window that opens at launch no longer leaves its tabs, workspaces and splits in memory. After #390, every other window was freed on close, but the launch window left one full set behind each time.

Three things held it:

1. Workspace menu (Sources/ProgramaApp.swift): the menu's button actions captured the window's tab manager and workspace, and SwiftUI keeps the menu around. The actions now look up the active tab manager when clicked, like the other menu items.
2. Sidebar resizer (Sources/ContentView+SidebarResizer.swift): teardown scheduled a deferred cursor release stored in view state. Once the view was gone, it never cleared, and it held the view and its tab manager. Teardown now releases the cursor right away.
3. Directory tracking (Sources/ProgramaConfig.swift): a Combine pipeline kept the last selected workspace. It now keeps only the workspace id and its directory publisher.

Measured

Tagged Debug build, heap counts of Workspace / TabManager / BonsplitController / SidebarState, 8 s after each close.

State	Before	After
Launch window + one new window, then close the launch window	3/3/3/2	2/2/2/2
3 cycles of reopen, new window, close the launch window	grew each cycle	2/2/2/2 every cycle

The one extra tab manager after a close is the spare that PrimaryTabManagerStore keeps for the next launch window, by design.

Test plan

• Workspace menu: Pin, Move Up, Move to Window and Close Workspace act on the frontmost window, including after closing and reopening the launch window.
• Dragging the sidebar resizer still shows and releases the resize cursor.
• Switching workspaces still updates the directory used for project config.