Add 2026 Frontier agenda (Atlas/Nexus/Compass) and per-stage speaker tabs; rename Mainstage to Plenary

[wilwixqa1]

Adds the Frontier stages to the 2026 Agentic AI Summit page, bringing it in line with the approved Option E mock. Purely additive to the existing layout; no repo structure or existing styling changed.

Agenda

• Renames the existing Mainstage agenda tabs to Plenary (Sat/Sun)
• Adds Frontier agenda tabs: Atlas (Sat/Sun), Nexus (Sat), Compass (Sat/Sun)
• New tabs reuse the existing timeline markup and showAgendaTab toggle

Speakers

• Adds Plenary/Atlas/Nexus/Compass filter tabs above the existing speaker grid
• Existing search box and card styling retained; filter combines stage + search
• Adds 102 Frontier speaker cards, sorted by talk length (15 then 10 then 5 min), ties by session order
• Removes Aditya Grover and Sanja Fidler from the Plenary grid (they are Atlas speakers, not on the Plenary agenda); both retain their Atlas cards

Assets

• Adds 99 new Frontier speaker headshots; the 3 pre-existing overlap images are left untouched

Verified locally: filter counts Plenary 51 / Atlas 29 / Nexus 21 / Compass 52, all 7 agenda tabs toggle correctly, no broken images, search and layout intact.