i18n(timeline): add zeitstrahl visual-fidelity strings (REQ-015)

Seven new timeline_* keys feeding the upcoming chrome: the header meta
line (grouping label + events count), the event-pill provenance token,
the ✉ sr-only label, the world-band "historisch" suffix, and the strip
density caption — present in de/en/es with matching key sets, pinned by a
new parity test.

Refs #833
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1046,6 +1046,13 @@
 	"timeline_derived_birth": "Geburt",
 	"timeline_derived_death": "Tod",
 	"timeline_derived_marriage": "Heirat",
+	"timeline_grouping_date": "Gruppierung: Datum",
+	"timeline_provenance_derived": "abgeleitet",
+	"timeline_provenance_curated": "kuratiert",
+	"timeline_letter_glyph_label": "Brief",
+	"timeline_layer_historical_suffix": "historisch",
+	"timeline_strip_density_caption": "Monats-Dichte",
+	"timeline_events_count": "{count} Ereignisse",
 	"event_editor_new_title": "Neues Ereignis",
 	"event_editor_edit_title": "Ereignis bearbeiten",
 	"event_editor_section_when": "Wann",