feat(05-01): create search types and utility functions

- Add SearchFilters interface for query, tags, type, dateRange
- Add defaultFilters constant with empty/default values
- Add hasActiveFilters() to detect when filters are active
- Add getDatePreset() helper for today/week/month presets

src/lib/types/search.ts

@@ -0,0 +1,52 @@
+export interface SearchFilters {
+	query: string;
+	tags: string[];
+	type: 'task' | 'thought' | 'all';
+	dateRange: {
+		start: string | null;
+		end: string | null;
+	};
+}
+
+export const defaultFilters: SearchFilters = {
+	query: '',
+	tags: [],
+	type: 'all',
+	dateRange: { start: null, end: null }
+};
+
+export function hasActiveFilters(filters: SearchFilters): boolean {
+	return (
+		filters.query.length >= 2 ||
+		filters.tags.length > 0 ||
+		filters.type !== 'all' ||
+		filters.dateRange.start !== null ||
+		filters.dateRange.end !== null
+	);
+}
+
+// Date preset helper for quick date range selection
+export function getDatePreset(preset: 'today' | 'week' | 'month'): { start: string; end: string } {
+	const now = new Date();
+	const end = now.toISOString().split('T')[0];
+
+	let start: Date;
+	switch (preset) {
+		case 'today':
+			start = now;
+			break;
+		case 'week':
+			start = new Date(now);
+			start.setDate(start.getDate() - 7);
+			break;
+		case 'month':
+			start = new Date(now);
+			start.setMonth(start.getMonth() - 1);
+			break;
+	}
+
+	return {
+		start: start.toISOString().split('T')[0],
+		end
+	};
+}