feat: restore tabs and splits at launch

The tab layout (tab order and names, pins, the active tab, split trees
with their ratios) is saved whenever it changes — checked once a second,
written only on change, so it survives a crash — and reopened at the
next launch: saved profiles reconnect and local shells start in the
directory they were in (on Linux the live /proc cwd, so it isn't one
command behind). Quick Connect tabs and placeholder panes are skipped.
A General setting turns it off; it never runs when launched with
--connect, --run-local or --open, and screenshot runs never save.

Verified: a local tab left in /usr/share came back there with a
'Restored 1 tab(s)' toast.

docs/overview.md

@@ -73,6 +73,7 @@ Use Nexus Terminal when you need to:
 - Shift+PageUp/PageDown/Home/End and the mouse wheel scroll the scrollback; on the alternate screen the wheel sends arrow keys, so `tmux`, `screen` and `byobu` scroll naturally.
 - Context menu with Copy, Paste, Select All, Find, Send Snippet, Port Forwards, Split Right, Split Down, Export Output, Clear and Reset.
 - Export the whole scrollback as plain text.
+- Tabs and split panes are restored at the next launch: connections reconnect and local shells reopen in their last directory.
 - Inline images in the iTerm2 format (`imgcat`, `wezterm imgcat`, `chafa -f iterm`…): PNG, JPEG and GIF, sized in cells, pixels or percent, scrolling with the text.
 - Shell integration (OSC 133): jump between prompts, copy the last command's output, and see each command's exit status and duration beside its prompt.
 - Status-bar latency for the active SSH session.