make every dialog close on ctrl+c, twice exits

[dgageot]

Pressing ctrl+c on any TUI dialog now opens the exit-confirmation dialog instead of quitting immediately (via HandleQuit / dialog-specific handlers). A second ctrl+c on the exit-confirmation exits, matching the existing two-press flow when no dialog is shown.

If the user cancels exit (N/Esc), the original dialog is restored — the exit-confirmation is stacked on top of it, not replacing it, so blocking dialogs (tool confirmation, elicitation, max iterations) don't leave the agent stuck waiting.

Behavior

State	1st ctrl+c	2nd ctrl+c
no dialog	opens exit confirmation	exits
any other dialog	stacks exit confirmation on top	exits
exit confirmation	exits	—

Implementation

• pkg/tui/tui.go — handleKeyPress intercepts ctrl+c before forwarding to the dialog manager. If the exit confirmation is already on top, the key is forwarded so it can exit via its own Yes binding; otherwise the exit-confirmation dialog is pushed onto the stack.
• pkg/tui/dialog/dialog.go — added Manager.TopIsExitConfirmation() so the top-level handler can route the key correctly.
• pkg/tui/dialog/elicitation.go — removed the now-redundant special-case that declined the elicitation and called tea.Quit on ctrl+c.
• pkg/tui/tui_ctrlc_test.go — three unit tests covering the table above.