Add Chart.js-based chart renderer and enforce immediate visual output

[Abdulmuiz44]

Motivation

• Provide a first-class, always-rendering chart pipeline by switching the client chart renderer to Chart.js so visuals requested by users reliably render in-browser.
• Ensure the LLM-driven visual workflow produces a visual immediately when the user explicitly asks for one, aligning system prompts across client and backend.

Description

• Replaced the previous Recharts-based renderer with a Chart.js canvas implementation in components/visuals/ChartRenderer.tsx, including Chart.js registration, chart-type normalization, data/series inference, nested-series flattening, themed options, and canvas export support (toBase64Image).
• Updated system prompt behavior in lib/mistral.ts and backend-server/src/services/mistral.ts so an explicit user request for a visual causes the model to generate the visual immediately in the required json:tera-ui format.
• Added chart.js to package.json and updated the lockfile (pnpm-lock.yaml) so Chart.js is installed and available to the client renderer.
• Preserved download/export UX by switching the download implementation to Chart.js canvas export and kept the existing chart configuration compatibility (line/bar/area/pie/radar/scatter/composed aliases).

Testing

• Ran type checking with pnpm run type-check, which completed successfully.
• Installed Chart.js with pnpm add chart.js@^4.4.3 (lockfile shows chart.js@4.5.1) and validated the dependency was added to package.json and pnpm-lock.yaml.
• Ran lint with pnpm run lint, which failed due to an unrelated configuration error: Invalid project directory provided, no such directory: /workspace/Tera/lint.

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