Design system adoption.

Changed
- The dashboard adopts the new design's palette and collapses the dark theme
  onto it. A cyan accent (#00B4E6 dark / #00758F light) replaces the orange,
  with five surface planes and five semantic states per theme. Hex literals in
  real declarations drop from 105 to 2; body.dark-theme component rules from
  53 to 3. Every text pair clears WCAG AA and every control boundary clears
  3:1, verified in a running dashboard in both themes.
- Type and density follow the design's scale: metric tiles are an uppercase
  micro-label over one large tabular number, on a six-step scale topping out
  at 24px. Every font-size and border-radius comes from a token.
- Focus rings restored. The vendored .btn:focus rule stripped every focus
  indicator with no replacement.
- Stack traces and pprof output render at a fixed size on a recessed surface.
  The previous rule asked for a 'Fira Code' that is not shipped.
- Table numerics use tabular-nums, so live columns stop jittering on each poll.

Added
- Design token layer: 122 custom properties in matched light and dark sets.

No breaking changes. See CHANGELOG.md.