feat(history): add session recording and history viewer

Explicit start/stop recording sessions (tray Start/Stop Recording) replace
the old continuous daily-rolling CSV log. A new Session History window
lists past sessions and charts CPU/GPU/RAM/net/disk/ping with egui_plot,
including hover tooltips and axis/cursor-linked charts so a moment in
time lines up across all metrics. Sessions can be pinned, renamed, and
deleted; the wallpaper host process picks up an active session from the
on-disk index too, since it can be the active poller instead of the main
app.

First iteration - more to come.

Refs #175