feat: add a history panel toggle

The history panel now hides, on the ☰ button in the top bar and on Ctrl+\.

Its width is captured before hiding and reapplied on show, so toggling does
not snap the panel back to a default. Both the width and whether it was
hidden persist between runs.

Layout state lives in state.ini beside the config rather than at QSettings'
default path, which keeps it obvious where it is and lets the checks point
it somewhere temporary instead of writing to the real one.

The button's checked state, not isVisible(), is the authority for whether
the panel is shown: isVisible() is False for every child of a window that
has not been mapped yet, so consulting it during startup would disagree
with what the user sees.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

0 files changed, 0 insertions, 0 deletions