fix(sidebar): declutter session items — drop message count, model, and source-tag badges (v0.50.64)

Squash-merges PR #584 by @aronprins.

Drops the meta row (message count, model slug, source-tag badge) from every sidebar session item. Each session now renders as a single title line — visible session count roughly doubles at typical viewport height.

Changes merged verbatim from contributor branch, plus maintainer additions:
- CHANGELOG entry for v0.50.64
- Version badge bump to v0.50.64
- New test: test_relative_time_today_bucket (closes minor coverage gap from code review)

Co-authored-by: aronprins <aronprins@users.noreply.github.com>

tests/test_session_sidebar_relative_time.py

@@ -76,14 +76,12 @@ def test_session_sidebar_js_has_dynamic_relative_time_helpers():
 def test_session_sidebar_renders_relative_time_and_meta_rows():
     # session-time element was removed from sessions.js in v0.50.40 to
     # give session titles full width — the CSS class is kept but set to display:none.
-    assert "session-time" not in SESSIONS_JS or True  # intentionally removed from JS
-    assert "session-meta" in SESSIONS_JS
+    # session-meta / metaBits were removed when we dropped message-count, model, and
+    # source-tag badges from the sidebar (design round 2).
     assert "orderedSessions" in SESSIONS_JS
     assert ".session-time" in STYLE_CSS
-    assert ".session-meta" in STYLE_CSS
     assert ".session-title-row" in STYLE_CSS
     assert ".session-item.active .session-title" in STYLE_CSS
-    assert "metaBits.join(' · ')" in SESSIONS_JS
     assert "|| _sessionTimeBucketLabel" not in SESSIONS_JS
     assert "const ONE_DAY=86400000;" not in SESSIONS_JS