fix: mobile terminal back and X buttons navigate to /sessions

Terminal sessions are opened in a new tab via window.open with
noopener,noreferrer. In a new tab, window.history has no previous
entry, so navigate(-1) silently does nothing. Both the back arrow
and the X button in the mobile terminal overlay called navigate(-1),
which made them appear broken.

Change both buttons to navigate('/sessions') so they always exit
to a sensible page regardless of how the terminal was opened.

Quality gates: tsc --noEmit (pass), build (pass)

apps/web/src/pages/TerminalPage.tsx

@@ -318,7 +318,7 @@ export const TerminalPage: React.FC = () => {
 						<div className="mobile-terminal-toolbar-left">
 							<button
 								className="mobile-terminal-toolbtn"
-								onClick={() => navigate(-1)}
+								onClick={() => navigate("/sessions")}
 								type="button"
 								aria-label="Back"
 							>
@@ -353,7 +353,7 @@ export const TerminalPage: React.FC = () => {
 							</button>
 							<button
 								className="mobile-terminal-toolbtn"
-								onClick={() => navigate(-1)}
+								onClick={() => navigate("/sessions")}
 								type="button"
 								aria-label="Exit terminal"
 							>