feat: download and copy-as-code for generated components

[GeneralJerel]

Summary

• Add download as HTML and copy-as-code buttons to all generated components (widgets, bar charts, pie charts)
• Downloaded files are standalone HTML that preserves animations, interactivity, and theming — open directly in a browser
• Copy-as-code copies the raw HTML fragment (widgets) or standalone HTML (charts) to clipboard
• Zero new dependencies — Chart.js loads from CDN in exported chart files, download uses native Blob API

Changes

New: apps/app/src/components/generative-ui/export-utils.ts

• assembleStandaloneHtml() — wraps widget HTML in a full document with theme CSS, import maps, and stubbed bridge functions
• chartToStandaloneHtml() — generates standalone Chart.js HTML from structured chart data
• triggerDownload() — browser file download via Blob + object URL
• slugify() — filename helper

Modified: apps/app/src/components/generative-ui/save-template-overlay.tsx

• Added download (arrow icon) and copy (clipboard icon) buttons alongside existing "Save as Template"
• Buttons appear on all components when content is ready, including when a template badge is shown
• Copy shows a check icon confirmation for 1.8s

Modified: apps/app/src/components/generative-ui/widget-renderer.tsx

• Exported SVG_CLASSES_CSS and FORM_STYLES_CSS constants (needed by export-utils)

Updated: .chalk/plans/demo-gallery.md

• Updated gallery plan with Variant-style masonry layout direction from Rework templates into a component gallery #55
• Added download dependency context

Closes

• Closes Save / Download Rendered Asset #14 — Save / Download Rendered Asset
• Closes Add copy-as-code and download options for generated artifacts #42 — Add copy-as-code and download options for generated artifacts
• Enables Rework templates into a component gallery #55 — Gallery items can now be downloaded

Test plan

• Generate a widget visualization (e.g. "how does a plane fly") → click download → open .html in browser → verify animations and theme work
• Generate a bar chart → click download → open in browser → verify Chart.js renders with animations
• Generate a pie chart → same check
• Click copy on a widget → paste in editor → verify raw HTML fragment
• Click copy on a chart → paste → verify standalone HTML
• Verify "Save as Template" still works alongside new buttons
• Verify buttons appear on matched-template components too

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