Add Python backend for SVG rendering, connect frontend to backend, and UI/usability improvements

[afffe18]

Motivation

• Provide a minimal Python backend endpoint to render plot SVGs and support Excel imports so the frontend can offload rendering and column extraction to a server.
• Improve editor ergonomics with tab renaming, automatic tab naming, URL-linked selection, theme settings and a simple menu to streamline configuration workflows.
• Replace the client-only Plotly rendering flow with a backend-rendered SVG preview to simplify rendering and enable server-side logic.
• Make configuration export deterministic for the backend by exposing toExternalConfig for JSON payloads.

Description

• Add a Flask backend at backend/app.py with endpoints POST /api/render-plot (accepts JSON config and returns placeholder SVG) and POST /api/import-database (accepts JSON or file uploads and extracts column names using openpyxl via _extract_columns_from_xlsx), and include backend/requirements.txt with flask and openpyxl.
• Wire the frontend to the backend by exporting toExternalConfig from src/utils/configIo.ts and updating src/components/PlotPage.tsx to send the config to /api/render-plot, display returned SVG markup, and show render state/errors.
• Add multiple UI/UX enhancements in src/App.tsx including theme support, menu/settings/about modals, tab rename and middle-click-to-open, automatic generated names via getNextTabName, URL query param synchronization for dataframe and frame, import validation against known columns, and dynamic whitelist keyword options.
• Adjust layout and asset pipeline: change #root sizing and dark-mode border in src/index.css, add a backend run script to package.json, and configure a dev proxy to the backend in vite.config.ts.

Testing

• No automated tests were run for this change.

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Codex Task

[afffe18]

@copilot resolve the merge conflicts in this pull request