fix(ui): use De Gruyter long arrows for swap button and timeline entries

Swap button: stack right/left arrows vertically at h-3.5 for a
compact look. Timeline: replace → ← text with Long-Arrow icons on
each letter entry and the distribution bar summary.

Refs: #179

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/briefwechsel/ConversationTimeline.svelte

@@ -84,8 +84,24 @@ const newDocUrl = $derived(
 		aria-label="Briefverteilung in diesem Zeitraum: {outCount} von {senderName ?? ''}, {inCount} von {receiverName ?? ''}"
 	>
 		<div class="flex justify-between text-sm font-bold">
-			<span class="text-primary">{outCount} von {shortSenderName} →</span>
-			<span class="text-accent">{inCount} von {shortReceiverName} ←</span>
+			<span class="inline-flex items-center gap-1 text-primary"
+				>{outCount} von {shortSenderName}
+				<img
+					src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Long-Arrow/Long-Arrow-Right-MD.svg"
+					alt=""
+					aria-hidden="true"
+					class="inline h-3.5 w-3.5 opacity-60"
+				/></span
+			>
+			<span class="inline-flex items-center gap-1 text-accent"
+				>{inCount} von {shortReceiverName}
+				<img
+					src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Long-Arrow/Long-Arrow-Left-MD.svg"
+					alt=""
+					aria-hidden="true"
+					class="inline h-3.5 w-3.5 opacity-60"
+				/></span
+			>
 		</div>
 		<div class="flex h-[5px] overflow-hidden rounded-full bg-line">
 			<div class="h-full bg-primary transition-all" style="width: {outPct}%"></div>
@@ -115,14 +131,14 @@ const newDocUrl = $derived(
 			class:border-l-primary={isOut}
 			class:border-l-accent={!isOut}
 		>
-			<span
-				class="w-[16px] shrink-0 text-sm font-black"
-				class:text-primary={isOut}
-				class:text-accent={!isOut}
+			<img
+				src={isOut
+					? '/degruyter-icons/Simple/Medium-24px/SVG/Action/Long-Arrow/Long-Arrow-Right-MD.svg'
+					: '/degruyter-icons/Simple/Medium-24px/SVG/Action/Long-Arrow/Long-Arrow-Left-MD.svg'}
+				alt=""
 				aria-hidden="true"
-			>
-				{isOut ? '→' : '←'}
-			</span>
+				class="h-4 w-4 shrink-0 opacity-60"
+			/>
 
 			<div class="min-w-0 flex-1">
 				<div class="mb-[2px] truncate text-sm font-bold text-ink">