feat(#252): add Line and Area chart style controls

[BorisTyshkevich]

What & why

Closes #252.

Adds a compact renderer-independent style contract for Line and Area panels:

• Clean, Smooth, Stepped, and Points presets in the chart configuration bar
• deterministic automatic marker visibility from final rendered data density
• explicit safe Chart.js curve, point-interaction, and line-width mappings
• lossless style persistence through panel switches, Library documents, shares, OAuth restoration, Dashboard tiles, and existing shared render paths
• canonical/generated schema documentation with forward-compatible unknown string and extension preservation

Existing charts without style remain valid and now resolve to linear + auto.

Verification

• npm test (100 files, 2,788 tests; per-file coverage gate passes)
• npm run build
• npm run test:e2e (120 tests; Chromium, Firefox, WebKit)

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency (or it's a deliberate, justified addition — see CONTRIBUTING)
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it