fix: rewrite session detail tests to match SSR template

The session_detail.html template is fully server-side rendered — it does a real DB lookup and returns HTTP 404 for unknown sessions, and renders metadata (intent, location, duration, participants, commits, notes) from the database rather than delegating to client-side JS.

All 10 tests now: - Create a real repo + session via _make_repo/_make_session helpers so the SSR route can find the data. - Assert on elements actually rendered by the Jinja2 template: meta-label/meta-value rows, Participants sidebar, commit-pill class, live/ended badge, session_id[:8] in heading, notes text, etc. - test_session_detail_404_marker → test_session_detail_404_for_unknown_session: updated to assert HTTP 404 (correct for SSR) instead of 200 + error JS. - Replaced six stale comment-section/JS-function tests (comments-section, loadComments, submitComment, renderComments, new-comment-body) with tests covering SSR-rendered fields: intent, location, meta-label layout, live badge, ended badge, closing notes.