fix: prevent tmux flicker on restart by matching existing window size

[aakhter]

Summary

When a PTY client re-attaches to an existing tmux session, the PTY size is hardcoded to 120x40, so tmux resizes the window to match the smaller dimensions. The xterm.js client then immediately resizes back to its actual viewport on the next render tick — every restart causes a visible flicker and loses one repaint of scrollback content.

Two small changes fix this:

• src/session.ts — query the existing tmux window size via tmux display -t <muxName> -p '#{window_width} #{window_height}' before pty.spawn, fall back to 120x40 only if the call fails.
• src/tmux-manager.ts — drop the hardcoded -x 120 -y 40 from tmux new-session so the initial size matches the first attaching client.

execFileSync is used (not execSync) so the muxName isn't shell-interpolated.

Test plan

• Start a session, wait for buffer to fill (>40 rows of output).
• Hard-reload the page (Ctrl+Shift+R).
• Observe: no flicker, no buffer truncation.
• Restart with a non-default xterm size (e.g. wide window): on attach the PTY/tmux are sized to match.
• Kill tmux mid-attach (corner case): falls back to 120x40 gracefully (catch block in the try { execFileSync … }).

🤖 Generated with Claude Code

[Ark0N]

Thanks for the fix! Querying the existing tmux size before attach is exactly the right shape, and the execFileSync + bounded fallback is a nice touch. Merged.