Restyle the dashboard to the Lightwell design language

Same tokens, sidebar shell, components and voice as Hammerhead: teal accent,
warm neutrals, status badges with text, dialogs with focus trapping that name
what they affect, empty states, skeleton loading, outline icons, 44px touch
targets and a slide-in drawer on phones. The strict CSP stays: no inline
styles or scripts, so icons and sizes use classes.

Adds docs/design.md and scripts/lint-design.mjs (run in CI), capitalizes
server error messages in one place, and rewrites interface copy to the voice
rules (sentence case, buttons that name the result, no dashes).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -168,6 +168,10 @@ Prometheus: `megalodon_server_up`, `megalodon_players`, `megalodon_players_max`,
   report that ID.
 - ARK:SE only (it is untested on ASA, which uses the same protocol but may differ in detail).
 
+## Design
+
+The dashboard follows the Lightwell design language, the same one Hammerhead uses: teal accent, warm neutrals, a sidebar, status badges with text, and dialogs that name what they affect. See [docs/design.md](docs/design.md). `node scripts/lint-design.mjs` checks the rules a machine can check.
+
 ## Development
 
 ```sh