Prepare 0.1.2 patch release

[Chenglong-MS]

Summary

• Bump flint-chart and flint-chart-mcp to 0.1.2, with MCP depending on flint-chart ^0.1.2.
• Keep the public Flint title as Flint: A Visualization Language for the AI Era, with surrounding copy explaining the semantic chart-spec layer.
• Harden MCP validate_chart preflight validation for malformed semantic specs before assembly.
• Fix Vega-Lite render dimension reporting to match root SVG/PNG artifact dimensions.
• Polish the public site mobile experience across landing, gallery, docs, and editor routes.

Validation

• npm run typecheck
• npm run lint (0 errors; existing warnings remain)
• npm run test
• npm run build
• npm run site:build
• npm run build -w site after the title update
• git diff --check
• VS Code diagnostics on touched TS/CSS files

Release Notes

• 0.1.2 is for the MCP/runtime fixes and site polish, not for a title-only change.
• Verified flint-chart@0.1.2 and flint-chart-mcp@0.1.2 are not yet published on npm.
• Untracked paper/blog artifacts are intentionally excluded from this PR.