Refine dark mode UX, menu layout, axis editor and plot download workflows

[afffe18]

Motivation

• Improve the UI/UX around dark/light mode and the JSON editor so the editor is readable in both themes.
• Restructure the top menubar to group actions (menu, import/export/reset, config/json, preview/create) for easier access.
• Make axis editing more ergonomic by moving name/label/mode to the left and putting axis columns on the right with an expand/collapse control.
• Provide clearer plotting workflows by adding per-plot download buttons while keeping download-all behavior and surface database import status.

Description

• Reworked the header/menu layout and grouped controls to match the requested structure; replaced some text labels with Preview one Plot / Create all Plots (changes in src/App.tsx).
• Added an axis editor redesign so the left column contains name, label, mode and the right column shows axis columns with an expand/collapse button via an updated MultiSelectInput and per-axis expanded state (changes in src/App.tsx).
• Added a per-dataframe database import status indicator that updates after successful imports and stores minimal per-dataframe status (changes in src/App.tsx).
• Improved JSON editor overlay and caret styling so syntax overlay and caret use appropriate colors in dark and light themes (resolvedDarkMode logic and adjusted pre/textarea classes in src/App.tsx).
• Added single-plot download buttons above each generated plot and preserved the existing “download all” and combined-HTML downloads (changes in src/components/PlotPage.tsx).
• Expanded README with a concrete Docker run guide showing a two-container (frontend + backend) workflow, network, and verification steps (README.md).

Testing

• Ran npm run lint and addressed lint warnings; npm run lint passed.
• Built the production bundle with npm run build; build completed successfully.
• No automated unit/test suites were modified; existing CI/test targets remain unchanged.

---

Codex Task