feat: add color support for line annotations

- Add color field to annotations schema with default blue (#3b82f6)
- Add color picker UI with 5 preset colors (red, green, blue, yellow, white)
- Pass selected color through component hierarchy (Page -> Toolbox, CandleChart -> SvgOverlay)
- Store color when creating line annotations
- Render lines with their stored color
- Update database with color column
- Preview lines show selected color during drawing

Phase 1 of LINE_DRAWING_IMPROVEMENTS.md complete

src/lib/db/schema.ts

@@ -14,5 +14,6 @@ export const annotations = sqliteTable('annotations', {
   timestamp: integer('timestamp').notNull(),
   label_type: text('label_type').notNull(),
   geometry: text('geometry'), // JSON string for line coordinates
+  color: text('color').default('#3b82f6'), // hex color code
   created_at: integer('created_at').notNull(),
 });