fix(web player + stats modal): no auto-resume after user pause; stable stats modal

Player: opening the PWA no longer auto-plays when the user had paused. Pausing via
the media notification while backgrounded routes through togglePlay, which now sets
S._userPaused; _onForeground only auto-resumes an INVOLUNTARY background stop
(_wasPlayingBeforeHide && !_userPaused). playCurrent clears the flag on a new track.
Involuntary-stop/freeze recovery on foreground is preserved.

Stats modal: no more small→big flash when switching period/tab — the body keeps a
stable min-height and retains its current content until the new data arrives (only
the first open shows 'Lade…'); a per-request token drops out-of-order responses.
SW cache -> ns-v71.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/state.js

@@ -15,6 +15,7 @@ export const S = {
   _scrubbing: false,
   _msPosLast: 0,
   _trackChanging: false,
+  _userPaused: false,   // true when the user deliberately paused → foreground must NOT auto-resume
   queueOpen: false,
 
   // Now Playing overlay