Migrate backend to FastAPI and improve plot builder UX

[afffe18]

Motivation

• Replace the untyped Flask placeholder backend with a modern FastAPI service that can receive JSON and generate a matplotlib SVG preview.
• Improve the plot-builder UI ergonomics: allow reordering tabs, move close controls inside tab outlines, fix theme toggle behavior, and add helpers requested for material color management.

Description

• Rewrote the backend from Flask to FastAPI and switched the startup script to use uvicorn backend.app:app; added a typed RenderPlotRequest model and a /api/render-plot handler that generates and returns a dummy matplotlib SVG built from the posted config.
• Kept and adapted /api/import-database to accept both JSON and multipart/form file uploads and to extract column headers from .xlsx using openpyxl.
• Implemented drag-and-drop reordering for dataframe tabs and frame tabs using DOM drag events and a safe moveItem helper.
• Moved the tab close button into each tab outline and updated the tab markup to improve hit targets and keyboard activation.
• Fixed theme toggling by applying html.style.colorScheme in addition to toggling the dark class so the UI respects the chosen theme.
• Renamed the top-right toggle from "Show Plot" to "Preview Plot" and updated the plot page heading/button text to match.
• Added a Generate colors button and confirmation modal to the Material colors section and implemented an HSV-based generator that spaces hues evenly and alternates brightness for maximal distinction.
• Updated backend Python dependencies and package scripts to support FastAPI/uvicorn/matplotlib and added python-multipart for form parsing.

Testing

• Built the client with npm run build which completed successfully.
• Validated the backend Python syntax with python3 -m py_compile backend/app.py which succeeded.
• Sanity-checked that the new /api/render-plot returns an SVG and that /api/import-database accepts both JSON and file form variants during local validation (no browser screenshot captured in this environment).

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Codex Task