feat(documents): badge undated rows instead of a bare em-dash

DocumentRow rendered a bare em-dash for null-dated letters — a glyph a
screen reader announces as nothing. Both breakpoints now render the single
DocumentDate component unconditionally (no {#if}/—/{:else}), so the cue
cannot drift; its unknown state is a neutral metadata chip ("Datum
unbekannt", text-ink-3, ≥4.5:1 both themes) with a non-color calendar glyph,
never red/amber. Present dates render at honest precision via
formatDocumentDate ("Juni 1916", not a fabricated day).

Refs #668

frontend/src/lib/document/DocumentRow.svelte

@@ -168,16 +168,12 @@ function safeTagColor(color: string | null | undefined): string {
 						     document DETAIL page, never in list/search rows — list rows surface only the
 						     honest label to keep scan-rows compact. showRaw={false} enforces this; the
 						     DocumentListItem payload also intentionally omits metaDateRaw. -->
-						{#if doc.documentDate}
-							<DocumentDate
-								iso={doc.documentDate}
-								precision={doc.metaDatePrecision}
-								end={doc.metaDateEnd}
-								showRaw={false}
-							/>
-						{:else}
-							—
-						{/if}
+						<DocumentDate
+							iso={doc.documentDate}
+							precision={doc.metaDatePrecision}
+							end={doc.metaDateEnd}
+							showRaw={false}
+						/>
 					</div>
 					<div class="flex items-start gap-2">
 						<ProgressRing percentage={item.completionPercentage} />