vendor Tailwind CSS, embed all static assets in binary

Remove CDN dependency (cdn.tailwindcss.com) and replace with a
pre-built, minified Tailwind CSS file embedded in the Go binary
via go:embed.

Changes:
- Add static/static.go with go:embed for css/ directory
- Add static/css/tailwind.min.css (9KB, contains only classes
  used by the dashboard template)
- Remove <script src="https://cdn.tailwindcss.com"> and inline
  tailwind.config from dashboard.html
- Replace with <link rel="stylesheet" href="/s/css/tailwind.min.css">
- Mount /s/ route for embedded static file serving (go-chi)
- Add /.well-known/healthcheck endpoint per GO_HTTP_SERVER conventions

Zero external HTTP requests from the browser. All assets served
from the binary itself.

Closes #82

static/static.go

@@ -0,0 +1,10 @@
+// Package static provides embedded static assets.
+package static
+
+import "embed"
+
+// Static contains the embedded static assets (CSS, JS) served
+// at the /s/ URL prefix.
+//
+//go:embed css
+var Static embed.FS