fix(frontend): improve PDF zoom and diff readability

- PDF viewer: append #zoom=page-width to iframe src so A4 letters fill
  the panel width instead of leaving large grey gutters
- Diff view: trim unchanged context to 4 words either side of each
  change, replacing long runs with '…' so edits are easy to spot

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -112,6 +112,44 @@ function personLabel(p: { firstName: string; lastName: string }): string {
 	return `${p.firstName} ${p.lastName}`.trim();
 }
 
+const DIFF_CONTEXT_WORDS = 4;
+
+type DiffPart = { value: string; added?: boolean; removed?: boolean };
+
+function trimContextParts(parts: DiffPart[]): DiffPart[] {
+	return parts.flatMap((part, i) => {
+		if (part.added || part.removed) return [part];
+		const tokens = part.value.split(/(\s+)/).filter(Boolean);
+		const wordCount = tokens.filter((t) => /\S/.test(t)).length;
+		if (wordCount <= DIFF_CONTEXT_WORDS * 2) return [part];
+
+		function keepFirst(n: number): string {
+			let count = 0;
+			const out: string[] = [];
+			for (const t of tokens) {
+				out.push(t);
+				if (/\S/.test(t) && ++count >= n) break;
+			}
+			return out.join('');
+		}
+		function keepLast(n: number): string {
+			let count = 0;
+			const out: string[] = [];
+			for (const t of [...tokens].reverse()) {
+				out.unshift(t);
+				if (/\S/.test(t) && ++count >= n) break;
+			}
+			return out.join('');
+		}
+
+		const isFirst = i === 0;
+		const isLast = i === parts.length - 1;
+		if (isFirst) return [{ value: '… ' + keepLast(DIFF_CONTEXT_WORDS) }];
+		if (isLast) return [{ value: keepFirst(DIFF_CONTEXT_WORDS) + ' …' }];
+		return [{ value: keepFirst(DIFF_CONTEXT_WORDS) + ' … ' + keepLast(DIFF_CONTEXT_WORDS) }];
+	});
+}
+
 function buildDiff(older: SnapshotDoc | null, newer: SnapshotDoc): DiffEntry[] {
 	const entries: DiffEntry[] = [];
 
@@ -119,7 +157,7 @@ function buildDiff(older: SnapshotDoc | null, newer: SnapshotDoc): DiffEntry[] {
 		const a = older?.[field] ?? '';
 		const b = newer[field] ?? '';
 		if (a === b) continue;
-		const parts = diffWords(a, b);
+		const parts = trimContextParts(diffWords(a, b));
 		entries.push({ kind: 'text', field, label: fieldLabels[field](), parts });
 	}
 
@@ -844,14 +882,14 @@ function versionLabel(v: VersionSummary, index: number): string {
 				</div>
 			{:else if fileUrl && doc.originalFilename.toLowerCase().endsWith('.pdf')}
 				<iframe
-					src={fileUrl}
+					src="{fileUrl}#zoom=page-width"
 					title={m.doc_preview_iframe_title()}
 					class="h-full w-full border-none bg-white"
 				></iframe>
 			{:else if fileUrl}
 				<div class="flex h-full w-full items-center justify-center overflow-auto p-8">
 					<img
-						src={fileUrl}
+						src="{fileUrl}#zoom=page-width"
 						alt={m.doc_image_alt()}
 						class="max-h-full max-w-full object-contain shadow-2xl"
 					/>