TerminalPane is a thin ref-based wrapper: xterm lifecycle lives in a
single mount effect keyed on sessionId, tearing the socket and terminal
down on cleanup. Preserves webglRenderer.ts's page-wide WebGL context
budget (acquire on mount, release on unmount, re-acquire on context
loss while still mounted) and reconnect.ts's exponential 500ms->10s
backoff. keys.ts's mobile modifier encoding moves into KeyBar.tsx.
Pane chrome (title, disconnect, sessions drawer) stays out of scope;
composition happens one level up.
