fix(dashboard): "Coach me" button — link to /health (GET route exists) not /explain

fix(dashboard): "Coach me" button — link to /health (GET route exists) not /explain

The Coach me button on the dashboard's Health insights widget linked
to GET /:username/:repo/explain, but only POST /:owner/:repo/explain
exists (src/routes/ask.tsx:406 — file-explainer JSON endpoint, wrong
shape for a button click). Result: clicking the button hit a 404 and
the back button got into a broken history state.

Fix: link to the existing GET /:owner/:repo/health page, which already
renders the health score breakdown + AI suggestions — exactly what
the button text and surrounding copy promise ("Health score X/100
— open the repo to see breakdown + AI suggestions").