feat(dashboard): [#304] implement per-metric detail pages and endpoint

[AaronProbha18]

Fixes: #304

Changes

• Added GET /session/<project_name>/<session_id>/metric/<metric_name> route (session_metric_detail) to fenn/dashboard/app.py, reusing scanner.get_session() — no scanner or type changes required.
• Route returns 404 when the session doesn't exist, and 404 when no <metric> points match metric_name for that session (there's no metric registry independent of recorded data, so an unknown name and a known-but-empty name both 404 — matches the acceptance criteria's "unknown metric name" wording).
• Added fenn/dashboard/templates/session_metric_detail.html, extending base.html with breadcrumb Overview › <project> › <session> › <metric_name>, mirroring session.html's #session-metrics-data JSON-blob pattern (scoped to the single metric's points instead of the full list) and its .chart-card/chart-svg markup so existing chart-rendering JS can pick it up unmodified.
• Added a "View details" link to each of the three .chart-card headers in session.html (train_loss, val_loss, acc), pointing at url_for('session_metric_detail', ...).

Notes

• No changes to scanner.py or types.py — get_session() already returns the full metrics list; filtering by metric_name happens in the route.