feat(frontend): ECharts quick chart from result grid #1899

Parent

#1851

Goal

Select numeric columns → bar/line chart via ECharts.

Done when

• Chart updates when selection changes

How to work

• Branch: issue/<n>-slug from epic/tauri-migration (create epic branch if missing).
• One PR → Closes #<n>.
• Target PR base: epic/tauri-migration until the SQLite slice is green.
• Do not edit Flutter app code unless this issue says so.