feat: customizable, resizable columns on ticket list, saved per view

- Column picker dropdown (grid icon next to sort/density)
  - Check/uncheck columns: ID, Subject, Status, Queue, Owner, Created, Updated
  - Subject column auto-expands (flex), others have fixed width
- Column resize handles: drag right edge of any column header
  - Min 50px, max 800px, body gets select-none during drag
- Columns persist with saved views (columns jsonb field)
- Reset to defaults when navigating away from a saved view
- Sticky column header row with muted background

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/lib/api.ts

@@ -268,7 +268,7 @@ export async function createView(data: {
   name: string;
   filters: { field: string; operator: string; value: string }[];
   sort_key?: string;
-  columns?: unknown[];
+  columns?: { key: string; label: string; width: number; visible: boolean }[];
   is_public?: boolean;
 }): Promise<{ data: SavedView | null; error: string | null }> {
   return request<SavedView>("/views", { method: "POST", body: JSON.stringify(data) });