feat(playlists): confirm before adding songs that are already in the playlist (#329)

Closes #327.

When every selected song is already in the target playlist, the
"Add to Playlist" flow used to silently show an "all skipped" toast.
Users could not tell whether they had hit the wrong target or whether
the action was deliberately ignored, and they had no way to add
duplicates intentionally (e.g. for a song they actually want twice).

New behavior: if every id is a duplicate, ask via the existing
GlobalConfirmModal — "Add anyway" appends them as duplicates,
"Cancel" keeps the previous silent-skip toast.

Mixed cases (some new, some duplicate) and the all-new path are
unchanged. Applied at all three add-to-playlist call sites in the
context menu (single/multi-track, album selection, artist selection).
Strings added to all 8 locales (en, de, es, fr, nl, nb, ru, zh).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/en.ts

@@ -1315,6 +1315,10 @@ export const enTranslation = {
     addSuccess: '{{count}} songs added to {{playlist}}',
     addPartial: '{{added}} added, {{skipped}} skipped (duplicates)',
     addAllSkipped: 'All skipped ({{count}} duplicates)',
+    addedAsDuplicates: '{{count}} songs added to {{playlist}} (as duplicates)',
+    duplicateConfirmTitle: 'Already in playlist',
+    duplicateConfirmMessage: 'All {{count}} songs are already in "{{playlist}}". Add them again as duplicates?',
+    duplicateConfirmAction: 'Add anyway',
     addError: 'Error adding songs',
     createAndAddSuccess: 'Playlist "{{playlist}}" created with {{count}} songs',
     createError: 'Error creating playlist',