feat(documents): add timeline helpers (boundary + gap-fill) (#385)

Pure utilities backing the TimelineDensityFilter component:
- monthBoundaryFrom/To convert YYYY-MM into LocalDate strings the existing
  /api/documents/search accepts (first/last day of the month).
- buildMonthSequence enumerates months between minDate and maxDate, crossing
  year boundaries.
- fillDensityGaps merges sparse backend buckets with the full month sequence,
  producing zero-count entries for months that the API omitted.

14 unit tests cover leap years, year boundaries, null inputs, and out-of-order
buckets.

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

frontend/src/lib/document/timeline.ts

@@ -0,0 +1,47 @@
+import type { components } from '$lib/generated/api';
+
+type MonthBucket = components['schemas']['MonthBucket'];
+
+export function monthBoundaryFrom(yearMonth: string): string {
+	return `${yearMonth}-01`;
+}
+
+export function monthBoundaryTo(yearMonth: string): string {
+	const [year, month] = yearMonth.split('-').map(Number);
+	const lastDay = new Date(Date.UTC(year, month, 0)).getUTCDate();
+	return `${yearMonth}-${String(lastDay).padStart(2, '0')}`;
+}
+
+export function buildMonthSequence(minDate: string | null, maxDate: string | null): string[] {
+	if (!minDate || !maxDate) return [];
+
+	const [minY, minM] = minDate.split('-').map(Number);
+	const [maxY, maxM] = maxDate.split('-').map(Number);
+
+	const sequence: string[] = [];
+	let year = minY;
+	let month = minM;
+
+	while (year < maxY || (year === maxY && month <= maxM)) {
+		sequence.push(`${year}-${String(month).padStart(2, '0')}`);
+		month += 1;
+		if (month > 12) {
+			month = 1;
+			year += 1;
+		}
+	}
+
+	return sequence;
+}
+
+export function fillDensityGaps(
+	buckets: MonthBucket[],
+	minDate: string | null,
+	maxDate: string | null
+): MonthBucket[] {
+	const sequence = buildMonthSequence(minDate, maxDate);
+	if (sequence.length === 0) return [];
+
+	const counts = new Map(buckets.map((b) => [b.month, b.count]));
+	return sequence.map((month) => ({ month, count: counts.get(month) ?? 0 }));
+}