Add frame/dataframe controls, tab drag hints, and closable alerts

[afffe18]

Motivation

• Improve editor UX by adding frame/dataframe management (duplicate, move) and a simple toggle for a dummy flag in the dataframe config.
• Make tab drag-and-drop more intuitive by rendering a visible placeholder at the prospective drop position.
• Fix material color editing and make notifications dismissible.
• Make backend xlsx column extraction align with the reference import code and document how to run the backend and reuse the reference plotter.

Description

• Added duplication and move operations: duplicateDataframe, duplicateFrame, and moveFrameToDataframe, plus UI controls (tab buttons and a Select + Move frame button) to trigger them; active frame/dataframe selection is updated after moves. (src/App.tsx)
• Improved drag-and-drop feedback by tracking dataframeDropIndex / frameDropIndex and rendering a dashed placeholder element at the hover position while dragging. (src/App.tsx)
• Added a dataframe-level dummy toggle that flips dataframes[i]._extensions.dummyValue from the UI via toggleDummyValue. (src/App.tsx)
• Fixed material colors UI: material keys are editable, each row has a remove ✕ button, and Add material color now uses getUniqueMaterialKey to generate unique keys (e.g. new_material, new_material_1). (src/App.tsx)
• Made alerts closable and updated the Alert component to accept children safely; added an inline ✕ to dismiss notifications. (src/components/ui/alert.tsx, src/App.tsx)
• Backend: replaced low-level openpyxl first-row parsing with pandas.read_excel(..., sheet_name=None) to extract sheet columns as dataframe column names, and added pandas to backend/requirements.txt. (backend/app.py, backend/requirements.txt)
• Documentation: clarified backend startup with uvicorn backend.app:app ... and documented how to call the reference plotter via reference/src/plot_ashby.py by calling main(dataframe_config, interactive=False). (README.md)
• Minor lint fix to remove an unused temporary variable and related small UI/behavior wiring. (src/App.tsx)

Testing

• Built the frontend with npm run build (Vite/TS build) and the build completed successfully.
• Ran linting with npm run lint (ESLint) and it completed with no errors after the fix.

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Codex Task