Phase 4: hand-rolled SVG charts (replace D3)

[CalebisGross]

Part of #339

Replace all D3.js charts with hand-rolled SVG/CSS. Zero external dependencies.

js/charts.js with shared helpers:

• createSVG(), scaleLinear(), scaleBand(), monotoneCubicPath() (Fritsch-Carlson), renderXAxis(), renderYAxis()

8 chart replacements (~660 lines total):

• Timeline sparkline (30-day bars) — Low
• SDK cost chart (bars + axes) — Low-Medium
• LLM token chart (stacked bars) — Medium
• Tool calls chart (bars + errors) — Low
• Memory Lifecycle (stacked area + curves) — High (~150 lines)
• KPI sparklines (inline SVG) — Low-Medium
• Signal Quality (horizontal bars) — Low
• Recall Learning Curve (connected dots) — Low-Medium

The D3 CDN dependency is a bug that breaks air-gap. This fixes it permanently.

[CalebisGross]

Completed on feat/dashboard-phase-0 branch. See commit history for details.