Fix relative file navigation — radicle-explorer

Navigating to a project.source view without defining a path, would skip the canonicalization of the path and end up returning /nodes/seed.radicle.garden/CONTRIBUTING.md which isn’t a valid link and would throw errors.

By instead of requiring this.#route to have a path, checking if this.#route.resource === "project.source we can fallback to README.md and don’t run into errors.

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       return `<a ${title ? `title="${title}"` : ""} href="${href.toLowerCase()}">${text}</a>`;
     }
     if ("path" in this.#route && !isUrl(href)) {
     if (this.#route.resource === "project.source" && !isUrl(href)) {
       href = routeToPath({
         ...this.#route,
         path: canonicalize(href, this.#route.path || "README.md"),