Hero pipeline-fan widget (replace pipeline matrix / new /pipelines/ landing)

## What

Replace (or augment) the source × target pipeline matrix on the landing page with a single converging-fan / Sankey-style diagram of all pipelines.

Sources (paper / CWL / Nextflow) on the left, targets (Galaxy / CWL) on the right; each pipeline is one fan-line connecting source → target. Hover a line and it previews phases inline (mini phase-graph tooltip). Click a line, navigate to the pipeline page.

## Why

The matrix encodes the design space cleanly but it's still a table. A fan diagram says \"Foundry is the funnel from many sources into Galaxy/CWL\" in one glance. Visually communicates the project premise better than prose.

## Scope when picked up

- Sketch in SVG (vanilla — no external lib, keep build slim) vs. d3-sankey vs. mermaid sankey. SVG by hand is probably enough at 5 pipelines.
- Decide whether this **replaces** the landing matrix, lives at `/pipelines/` as the listing experience, or both.
- Hover/touch interaction: phase-graph mini-preview popover. Reuse `PhaseGraph.astro` shrunken? Or a custom condensed form.
- Empty cells in the design space (e.g., CWL → CWL): show as ghost lines or hide.
- Mobile: fan doesn't fan well on narrow screens — collapse to the matrix or to a list?
- Accessibility: every fan-line needs a real keyboard-focusable link with proper aria-label.

## Related

- #2 (rich pipeline phase visualization) — this widget is the macro view, that issue is the micro view. Decisions probably want to land together.
- Landing redesign issue (option 1: source × target matrix) is the fallback / coexisting layout.