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>

internal/web/static/index.html

@@ -3,13 +3,13 @@
 <head>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
+<meta name="color-scheme" content="light dark">
 <title>Megalodon</title>
-<link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 32 32'%3E%3Cpath d='M2 22c6-2 9-9 14-14 1 5 4 8 14 14-6 0-9 2-12 6-2-3-6-5-16-6z' fill='%232a7fb5'/%3E%3C/svg%3E">
+<link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%231f5c5c' stroke-width='2' stroke-linecap='round' stroke-linejoin='round'%3E%3Cpath d='M3 20c4-1 7-5 8-14 3 5 8 7 10 14z'/%3E%3C/svg%3E">
 <link rel="stylesheet" href="style.css">
 </head>
 <body>
 <div id="app"></div>
-<div id="toasts" aria-live="polite"></div>
 <script src="app.js"></script>
 </body>
 </html>