feat: add shared Annotation, AnnotationType, Geometry interfaces in src/types/annotations.ts

Creates src/types/annotations.ts with typed interfaces matching actual
usage in CandleChart.tsx, page.tsx, and DB schema. Marks task 9.2 done.

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

src/types/annotations.ts

@@ -0,0 +1,35 @@
+/**
+ * Annotation types for point and line/rectangle annotations on charts.
+ *
+ * Geometry fields use Unix epoch seconds for time values.
+ * startTime / endTime correspond to candle timestamps.
+ * startPrice / endPrice correspond to price axis values.
+ */
+
+export interface Geometry {
+  startTime?: number;
+  startPrice?: number;
+  endTime?: number;
+  endPrice?: number;
+}
+
+export interface AnnotationType {
+  id: number;
+  name: string;
+  display_name: string;
+  color: string;
+  category: string; // 'marker' | 'line' | 'rectangle'
+  icon: string | null;
+  is_active: boolean;
+  created_at?: number | Date;
+}
+
+export interface Annotation {
+  id: number;
+  chart_id: number;
+  timestamp: number; // Unix epoch seconds
+  label_type: string;
+  color: string | null;
+  geometry: Geometry | null;
+  created_at: number | Date;
+}