Add bidirectional hover highlight between Gantt chart and table

Each Gantt bar and table row gets a data-conf-key attribute
(ISO start date + conference name). A small JS lookup map connects
them so hovering either element highlights both simultaneously:
- Gantt bar: filter brightness + inset white box-shadow
- Table row: yellow tint via background-color

Hovering a Gantt bar also scrolls the matching table row into view
(scrollIntoView nearest) so future conferences are reachable without
manual scrolling. The key field is pre-computed in
build_conference_timeline() to keep the template simple.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_view.py

@@ -482,6 +482,7 @@ def build_conference_timeline(
                 "lane": lane,
                 "left_pct": left_pct,
                 "width_pct": width_pct,
+                "key": f"{conf['start_date'].isoformat()}|{conf['name']}",
                 "label": (
                     f"{conf['name']}"
                     f" ({conf['start_date'].strftime('%-d %b')}–"