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>

public/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v70';
+const CACHE = 'ns-v71';
 const PRECACHE = ['/', 'index.html', 'manifest.json', 'icon.svg'];
 const SKIP_PATHS = ['/rest/stream', '/rest/download', '/config.js'];
 // Auth token params stripped from cache keys so artwork survives across sessions/token refreshes.