Restore UI language toggle and complete config editor controls

[afffe18]

Motivation

• Restore the original top-right UI language toggle as a UI-only control and separate it from the persisted dataframe/plot language setting to match previous UX expectations.
• Re-add missing editor controls from reference/docs/config_explanation.jsonc so the visual editor covers all guideline, layer, axis and material color settings.
• Improve grouping and input UX for source selection, plot languages and axis/whitelist selection while removing unused/duplicated code.

Description

• Restored a header EN/DE UI language toggle that is not persisted to the config and re-labeled the persisted language field as Dataframe language in the editor (src/App.tsx).
• Regrouped source & language controls into one section and moved import_sheet, Teable/API fields and plot language management there (src/App.tsx).
• Added full guideline controls and helper functions for adding/updating guidelines, including x, y, m, lineProps, fontsize, fontColor, label, labelAbove and labelPadding (src/App.tsx).
• Implemented material color editing with a color picker (type="color") plus a manual text input and an "Add color" action (src/App.tsx).
• Converted axis columns and layer whitelist keywords to placeholder multi-select dropdowns (temporary options) and added MultiSelectInput helper component (src/App.tsx).
• Enforced non-negative linewidth input for layers via min={0} and Math.max(0, ...) clamp, and exposed additional layer controls (alphaPoints, alphaAreas) (src/App.tsx).
• Added a reset confirmation modal so Reset opens a confirmation popup instead of immediately replacing config (src/App.tsx).
• Removed the now-obsolete DataSourceSection component and consolidated its behavior into the main editor (src/components/DataSourceSection.tsx deleted).
• Updated README.md to document the current app status and developer commands.

Testing

• Ran npm run lint after fixes (initial lint error was resolved) and the lint pass completed successfully.
• Built the production bundle with npm run build which completed successfully (no type/lint errors remained and the build artifacts were produced).
• No automated UI tests were run in this change set.

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Codex Task