Design tokens and layout cleanup

The styling is full of one-off arbitrary values (w-62.5, max-w-475, h-150, -top-48 -right-28 for dropdown positioning) and the z-index situation is anarchy: z-49, z-50, z-101, z-9997 through z-9999, plus a typo'd bg-transparentz-101 class in Dialog that does nothing. The stats bento grid is hand-placed col-start/row-start coordinates re-specified per breakpoint, which makes any layout change miserable.

• Define semantic tokens in the Tailwind @theme block: spacing scale, z-index scale (nav, dropdown, modal, toast), container widths
• Replace recurring arbitrary values with the tokens
• Rebuild the stats bento with named grid-template-areas per breakpoint
• Fix the dead bg-transparentz-101 class while there