[itscheems]

Articles now support inline and display formulas with editable LaTeX source and live previews. Formula content survives draft saving, reopening, publication, reading, and direct/HTTP transport round trips.

• Add article document schema 4 with bounded formula source, escaped HTML projections, and searchable source text. Existing document versions remain readable.
• Add formula insertion and editing controls, validation feedback, undo support, responsive rendering, and ordinary text entry after formula blocks.
• Keep article interaction feedback consistent: stable tooltip positioning, independent author-preview anchors, fullscreen-only outline navigation, and inactive formula selections without unsolicited highlighting.
• Reveal article theme changes from the appearance control, with direct switching for reduced motion or unsupported browsers.

Compatibility: no database migration is introduced. New formula documents require a schema-4-aware server and reader; older document versions remain supported.

Validation:

• mise exec -- go test ./..., mise exec -- go vet ./..., and mise exec -- golangci-lint run passed.
• The affected web selection covers 106 suites and 1,406 cases. Two legacy assertions were updated for independent author links and current-schema writes; their 64-case focused rerun passed, alongside the other 104 passing suites. Author routing and complete map-node content checks are retained.
• Web lint, build/type checking, changed-file formatting, and diff checks passed.
• Real-browser checks covered formula editing, cancellation, undo/redo, persisted reading, narrow-screen overflow, first/repeated tooltips, author previews, outlines, text carets after formulas, and both theme transition directions including reduced motion.

Full hosted CI will run on the pull request. Local release and deployment checks were omitted because this change does not alter deployment configuration or database migrations.

Closes #207