feat(i18n): add the timeline tag-chip theme label

timeline_tag_chip_label (de "Thema" / en "Topic" / es "Tema") is the sr-only
prefix the /zeitstrahl letter tag chip reads out so color is never the only
cue. Pinned per locale in messages.spec.ts; the tag name itself is rendered as
data, never translated.

Refs #835
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/de.json

@@ -1050,6 +1050,7 @@
 	"timeline_provenance_derived": "abgeleitet",
 	"timeline_provenance_curated": "kuratiert",
 	"timeline_letter_glyph_label": "Brief",
+	"timeline_tag_chip_label": "Thema",
 	"timeline_layer_historical_suffix": "historisch",
 	"timeline_strip_density_caption": "Monats-Dichte",
 	"timeline_events_count": "{count} Ereignisse",

frontend/messages/en.json

@@ -1050,6 +1050,7 @@
 	"timeline_provenance_derived": "derived",
 	"timeline_provenance_curated": "curated",
 	"timeline_letter_glyph_label": "Letter",
+	"timeline_tag_chip_label": "Topic",
 	"timeline_layer_historical_suffix": "historical",
 	"timeline_strip_density_caption": "Monthly density",
 	"timeline_events_count": "{count} events",

frontend/messages/es.json

@@ -1050,6 +1050,7 @@
 	"timeline_provenance_derived": "derivado",
 	"timeline_provenance_curated": "curado",
 	"timeline_letter_glyph_label": "Carta",
+	"timeline_tag_chip_label": "Tema",
 	"timeline_layer_historical_suffix": "histórico",
 	"timeline_strip_density_caption": "Densidad mensual",
 	"timeline_events_count": "{count} eventos",