Add font size controls in sidebar

[guysmoilov]

Problem\nThe terminal text size is fixed today. On smaller phones or for accessibility needs, users may need larger or smaller text to comfortably read and interact with tmux.\n\n## Proposal\nAdd font size controls in the UI, likely in the sidebar/drawer, so users can adjust terminal font size at runtime.\n\n## Suggested behavior\n- Provide at least increase/decrease controls (or a slider) in the sidebar\n- Apply changes immediately to the active xterm instance\n- Persist the chosen font size locally (e.g., localStorage) so it survives reloads\n- Define sensible min/max/default values\n\n## Acceptance criteria\n- User can change terminal font size from the sidebar\n- Font size updates are visible immediately without reconnecting\n- Setting persists across page reloads\n- Mobile layout remains usable with min and max values