fix(security): safe links and security headers (FLAW-28, FLAW-29)

safeHref lets only http, https and mailto become links (Markdown viewer,
outliner, legal links); javascript: and friends show as text. A
middleware sets a Content-Security-Policy (scripts only from MindSift,
no framing, connections only to this server), X-Frame-Options,
nosniff, Referrer-Policy and a Permissions-Policy. The two inline
scripts of index.html moved to files so script-src stays 'self'.

The second inline script also deleted every cache on each page load,
which undermined the offline start; it is gone.

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

internal/api/server.go

@@ -83,6 +83,7 @@ func (s *Server) Router() http.Handler {
 	r := chi.NewRouter()
 
 	r.Use(s.clientIPMiddleware)
+	r.Use(s.securityHeaders)
 	r.Use(s.demoResetGate)
 	r.Use(middleware.Recoverer)
 	r.Use(s.apiLoggingMiddleware)