feat(frontend): add precision-aware document date formatter

Adds formatDocumentDate — a pure, branch-per-precision label function that
renders a document date at exactly the precision the data claims (DAY → full
date, MONTH → "Juni 1916", SEASON → localized season word, YEAR → "1916",
APPROX → "ca. 1916", RANGE with collapse/expand/open-ended, UNKNOWN → "Datum
unbekannt"). Delegates to the existing date.ts helpers (shared T12:00:00
convention) and routes every localized word through Paraglide.

A shared docs/date-label-fixtures.json table is asserted by this spec and will
be asserted by the Java title formatter, as the drift guard requested in
review (Markus/Sara). Adds de/en/es precision/season/edit-form i18n keys.

Assumption: SEASON structured label is localized per locale (Decision 4),
with the verbatim raw cell preserved as a separate secondary line by callers.

Refs #666

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/en.json

@@ -261,6 +261,24 @@
 	"doc_preview_iframe_title": "Document Preview",
 	"doc_image_alt": "Original scan",
 	"doc_no_date": "No date",
+	"date_precision_unknown": "Date unknown",
+	"date_precision_approx_prefix": "c.",
+	"date_range_open_prefix": "from",
+	"date_season_spring": "Spring",
+	"date_season_summer": "Summer",
+	"date_season_autumn": "Autumn",
+	"date_season_winter": "Winter",
+	"date_original_label": "Original:",
+	"date_unknown_icon_label": "Date unknown",
+	"form_label_date_precision": "Date precision",
+	"form_label_date_end": "End date",
+	"date_precision_option_day": "Exact day",
+	"date_precision_option_month": "Month",
+	"date_precision_option_season": "Season",
+	"date_precision_option_year": "Year",
+	"date_precision_option_range": "Range",
+	"date_precision_option_approx": "Approximate",
+	"date_precision_option_unknown": "Unknown",
 	"person_merge_will_be_deleted": "will be deleted.",
 	"comp_typeahead_placeholder": "Type a name...",
 	"comp_typeahead_loading": "Searching...",