Add 'Refresh Tab' and 'Refresh Pane' to right-click context menus

[danshapiro]

Summary

Add two new context menu actions for refreshing terminals and panes:

1. "Refresh Tab" — right-click dropdown on each tab that calls refresh on every sub-pane in that tab's layout tree.
2. "Refresh Pane" — right-click dropdown on each pane that regenerates the pane content using the best available method for its type:
   • Browser panes: reload/refresh the embedded browser page
   • Terminal panes (coding agents like Claude Code, Codex): reset the WebSocket connection or re-attach to the underlying PTY session
   • Plain terminal panes: re-attach / reconnect to the PTY, replaying the scrollback buffer

Motivation

Currently there's no quick way to recover a pane that has gotten into a bad state (stale connection, frozen browser, stuck agent) without closing and re-creating it. A refresh action preserves the session while re-establishing the frontend connection.

Acceptance Criteria

• Right-clicking a tab shows a context menu with "Refresh Tab" (among existing items)
• Right-clicking a pane (e.g. pane header/chrome) shows a context menu with "Refresh Pane"
• "Refresh Pane" dispatches per-content-type logic:
  • Browser: reload the iframe/webview
  • Terminal (agent): detach + re-attach or equivalent reconnect
  • Terminal (shell): detach + re-attach, replaying scrollback
• "Refresh Tab" iterates all panes in the tab's layout tree and refreshes each
• Unit and integration tests cover the new menu items and refresh logic
• Accessible: menu items have proper labels and keyboard support