Redesign the dashboard and sign-in page with the Lightwell design language

Warm neutrals, teal accent, 15px type, and Lightwell's component shapes. Status is a badge with text
instead of a colored dot. Every destructive action asks first, naming the object and what is lost.
Dialogs trap focus, close on Escape and return focus. The sidebar is a drawer on phones, there is a
theme switch, loading shows skeleton rows, and empty states explain what belongs there. Emoji and
dashes are gone, and server errors are capitalized and tell visitors what to do next.
Adds scripts/lint-design.mjs and docs/design.md.

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

README.md

@@ -259,6 +259,10 @@ Blacktip redirects an unauthenticated visitor to Hammerhead's login page. Hammer
 token with a per-edge key and sends the visitor back; Blacktip verifies it and sets a cookie. The same
 decision function runs in both proxy modes.
 
+## Design
+
+The dashboard follows the Lightwell design language: warm neutrals, one teal accent, system fonts, 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 that a machine can check.
+
 ## Limits and notes
 
 - No client-to-site VPN (Pangolin's Olm/private resources) and no blueprints/YAML import.