feat(lyrics): add copy-to-clipboard button in sidebar header

A Copy icon button to the left of the Display settings trigger writes
the current track's lyrics (joined with newlines) to the clipboard via
the Tauri clipboard-manager plugin. The icon briefly swaps to a Check
for 1.5s on success, and a success/error toast is shown. The button is
disabled while lyrics are loading, when an error is displayed, or when
no lyrics are available.

cc

src/lib/i18n/locales/de.json

@@ -602,6 +602,9 @@
     "lyrics": "Songtexte",
     "lyricsControls": {
       "openControls": "Anzeigeeinstellungen",
+      "copyLyrics": "Liedtext kopieren",
+      "copySuccess": "Liedtext kopiert",
+      "copyError": "Liedtext konnte nicht kopiert werden",
       "title": "Anzeige",
       "autoFollow": "Auto-Scroll",
       "font": "Schrift",