Deck: bigger nav buttons, remove click-anywhere paging (keyboard arrows + nav buttons only)

Briefing deck: light theme, dated 2026-07-03, key figures per stage (snow/WB/dPL fields/routing)

Add PPT-style briefing deck (talk.html): 3-stage differentiable CREST, pycrest GPU, CONUS results incl. fresh Stage-3 routing, global path + discussion points