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/headers_test.go

@@ -0,0 +1,34 @@
+package api
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+func TestSecurityHeaders(t *testing.T) {
+	srv, _, _ := setupTestServer(t)
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/api/v1/health", nil)
+	req.Host = "notes.example.com"
+	srv.Router().ServeHTTP(rec, req)
+	csp := rec.Header().Get("Content-Security-Policy")
+	for _, want := range []string{"script-src 'self'", "frame-ancestors 'none'", "object-src 'none'", "wss://notes.example.com"} {
+		if !strings.Contains(csp, want) {
+			t.Errorf("CSP lacks %q: %s", want, csp)
+		}
+	}
+	if strings.Contains(csp, "unsafe-eval") || strings.Contains(strings.Split(strings.Split(csp, "script-src")[1], ";")[0], "unsafe-inline") {
+		t.Errorf("scripts must not allow inline or eval: %s", csp)
+	}
+	for k, v := range map[string]string{"X-Frame-Options": "DENY", "X-Content-Type-Options": "nosniff", "Referrer-Policy": "same-origin"} {
+		if rec.Header().Get(k) != v {
+			t.Errorf("%s = %q, want %q", k, rec.Header().Get(k), v)
+		}
+	}
+	// A Host header with quotes or semicolons must not be able to extend the policy
+	if p := contentSecurityPolicy("evil.com; script-src *"); strings.Contains(p, "evil.com") {
+		t.Errorf("host must not be copied into the policy unchecked: %s", p)
+	}
+}