Add SSH over bastion, local terminal, and terminal split improvements

[rishabhofficial]

• SSH over bastion/jump host: Full support for connecting through a bastion host with independent credentials (host, port, username, password or SSH key).
  Collapsible "Over SSH" section in the host edit modal with bastion credentials stored securely in OS keychain.

  • Local terminal: New portable-pty-based local shell sessions. "Local Terminal" option in the host picker dropdown spawns the user's default shell ($SHELL)
    without SSH. Supports split, resize, and disconnect like SSH sessions.
  • Terminal split fixes: Fixed missing onResizeRef/onResizeEndRef refs in use-resize-handle.ts. Terminal output now survives split remounts via output buffer
    replay. Host picker pane rendered in a proper panel with PaneHeader (split/zoom/close buttons).
  • Pending pane system: Cmd+T/Cmd+N opens a host picker in a new tab/split pane. Tabs correctly registered in unified tab store. Close button and split support
    on pending panes.
  • Bug fixes: Fixed undefined encoder in Terminal.tsx, fixed TerminalTabs.tsx reading from wrong store, fixed AppShell.tsx referencing non-existent
    setActivePage/activePage, fixed session-store using non-existent tabOrder/activeTabId properties.

  Test plan

  • Connect to a host with bastion configured — verify tunnel works
  • Save a host with "Over SSH" enabled using password auth, reconnect from saved hosts
  • Save a host with "Over SSH" enabled using SSH key auth, reconnect from saved hosts
  • Open a host picker via Cmd+(N/T) → "Local Terminal" button
  • Split a local terminal (Cmd+D) — verify both panes work independently
  • Split an SSH terminal — verify old terminal output is preserved
  • Close a pending host picker pane (X button and Cmd+W)