DamageMeter v0.2.8

v0.2.8: clear history pin on combat start + Live button on toolbar

After clicking View on a stored session, the main meter stayed stuck on
that historical view forever because nothing reset HistoryWindow's
PinnedSession. The Unpin button was hidden in the history-window header
where users couldn't find it.

Two fixes:

1. CombatTracker.OnSessionStarted is now wired in Plugin.cs to call
   HistoryWindow.ClearPin() on every new combat session. The moment
   the user swings again, the meter snaps back to live data.

2. MainWindow toolbar grows a "Live" button (amber) immediately
   to the left of Chart/Graph, but only when PinnedSession is set.
   Clicking it calls ClearPin() and the meter returns to live without
   having to open the History window.

ClearPin() is a new public method on HistoryWindow so external callers
(Plugin event subscriber, MainWindow toolbar) can drop the pin without
poking the property directly.