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/DocumentDate.svelte

@@ -28,13 +28,21 @@ const showRawLine = $derived(
 </script>
 
 <span class="inline-flex flex-col">
-	<span class="inline-flex items-center gap-1">
-		{#if isUnknown}
-			<!-- Non-color cue (WCAG 1.4.1): a calendar-with-question glyph. The visible
-			     "Datum unbekannt" text is the redundant textual cue, so the icon is
-			     decorative and hidden from assistive tech (per Leonie's a11y note). -->
+	{#if isUnknown}
+		<!--
+			Neutral metadata chip (#668): an undated letter is an absence, NOT an error,
+			so the chip is neutral (text-ink-3 on bg-surface, ≥4.5:1 in both themes) —
+			never red/amber. Matches the archive-metadata chip pattern in the row. The
+			non-color cue (WCAG 1.4.1) is the calendar-with-question glyph; the visible
+			"Datum unbekannt" text is the redundant textual cue and IS the accessibility,
+			so it is announced inline (never aria-hidden) while the icon is decorative.
+		-->
+		<span
+			data-testid="undated-badge"
+			class="inline-flex items-center gap-1 rounded border border-line px-1.5 py-0.5 font-sans text-[10px] tracking-widest text-ink-3 uppercase"
+		>
 			<svg
-				class="h-3.5 w-3.5 shrink-0 text-ink-3"
+				class="h-3 w-3 shrink-0 text-ink-3"
 				viewBox="0 0 24 24"
 				fill="none"
 				stroke="currentColor"
@@ -48,9 +56,13 @@ const showRawLine = $derived(
 				<path d="M9 16a1.5 1.5 0 0 1 3 0c0 1-1.5 1.2-1.5 2.2" />
 				<path d="M10.5 21h.01" />
 			</svg>
-		{/if}
-		<span>{label}</span>
-	</span>
+			{label}
+		</span>
+	{:else}
+		<span class="inline-flex items-center gap-1">
+			<span>{label}</span>
+		</span>
+	{/if}
 	{#if showRawLine}
 		<!-- Visible secondary line (WCAG 1.4.13 — not tooltip-only). raw is untrusted
 		     verbatim spreadsheet text; rendered via default Svelte interpolation, which