Arcane is an interface for managing Docker containers, images, networks, and volumes. Prior to 1.19.0, the unauthenticated GET /api/app-images/logo endpoint reflects a user-supplied color query parameter into the body of an SVG document via strings.ReplaceAll with no escaping. The substitution lands inside a <style> element of the embedded logo.svg, allowing an attacker to close the style block and inject executable <script> content. Because the response is served as image/svg+xml and Arcane sets no Content-Security-Policy or X-Content-Type-Options headers, navigating a logged-in admin victim to a crafted URL executes attacker-controlled JavaScript in Arcane's origin and rides the victim's HttpOnly JWT cookie to fully compromise the admin account. This vulnerability is fixed in 1.19.0.

Affected: Go: github.com/getarcaneapp/arcane/backend

## Summary

The unauthenticated `GET /api/app-images/logo` endpoint reflects a user-supplied `color` query parameter into the body of an SVG document via `strings.ReplaceAll` with no escaping. The substitution lands inside a `<style>` element of the embedded `logo.svg`, allowing an attacker to close the style block and inject executable `<script>` content. Because the response is served as `image/svg+xml` and Arcane sets no Content-Security-Policy or `X-Content-Type-Options` headers, navigating a logged-in admin victim to a crafted URL executes attacker-controlled JavaScript in Arcane's origin and rides the victim's HttpOnly JWT cookie to fully compromise the admin account.

## Details

The route is registered in `backend/internal/huma/handlers/appimages.go:53-61` with an explicitly empty security requirement, marking it as public:

```go huma.Register(api, huma.Operation{ OperationID: "get-logo", Method: http.MethodGet, Path: "/app-images/logo", ... Security: []map[string][]string{}, // explicit: no auth }, h.GetLogo) ```

`backend/internal/huma/middleware/auth.go:209-213` honors the empty `Security` value by returning `reqs.isRequired == false` and short-circuiting with `next(ctx)`, so no JWT/API-key check runs.

`GetLogoInput.Color` (`appimages.go:23`) is declared with no validation tags:

```go type GetLogoInput struct { Full bool `query:"full" default:"false" ...` Color string `query:"color" doc:"Optional accent color overri…