[FEATURE] Per-window/per-split independent font size setting

Problem
Font size (Cmd+/Cmd-) is currently a single global setting shared across the whole app — every window and split pane changes together, even when they're on completely different physical displays.

Use case
I regularly use this app on multiple displays at the same time, not sequentially:

• MacBook laptop screen
• Large external desktop monitor
• iPad (as a secondary display)

These three screens have very different physical sizes and viewing distances, so each needs its own comfortable font size — but since font size is a single global setting, all windows/panes across all displays are locked to the same size no matter what. There's no size that works well for a laptop screen up close AND a large monitor across the desk AND an iPad at the same time. (Relocating a single window between displays at different times is a secondary version of this same problem — but the main pain point is genuinely simultaneous multi-display use.)

Suggested behavior
Make font size a per-window (ideally per-split-pane) setting instead of a single global one:

• Each window/pane remembers and applies its own independent font size
• Cmd+/Cmd- while focused on one window/pane only affects that window/pane, not all open windows across all displays

This becomes more relevant now that the desktop redesign supports split view and multiple windows across multi-monitor + tablet setups — a common setup is exactly this kind of simultaneous multi-display use.