feat(web): protect launcher dashboard with token and SPA login (#1953)

Add token-based authentication for the Launcher's embedded Web Dashboard.

- Ephemeral token generated in-memory each run (or via PICOCLAW_LAUNCHER_TOKEN env var)
- HMAC-SHA256 session cookie (HttpOnly, SameSite=Lax, Secure when HTTPS)
- Bearer token support for API/script access
- Rate limiting on login (10 attempts/IP/min)
- Referrer-Policy: no-referrer on all responses
- POST-only logout with JSON content-type (CSRF-safe)
- System tray "Copy dashboard token" action
- Login page shows contextual help (console/tray/log file path)
- Path traversal protection via path.Clean
- X-Forwarded-Host/Port/Proto support for reverse proxy deployments
- Full i18n support (English, Chinese)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/config/security_integration_test.go

@@ -15,7 +15,7 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
-// Test JSON unmarshal of private fields
+// Test JSON unmarshal of private fields (unexported fields are never filled, with or without json tag).
 func TestJSONUnmarshalPrivateFields(t *testing.T) {
 	type testStruct struct {
 		PublicField  string `json:"public"`