refactor(routes):

- keep install-mode root redirect to /install and add tests covering install root redirect and install page serving
- change root backend handler from Any("/") to POST("/") so GET / can consistently serve the SPA after install

Co-authored-by: Copilot <copilot@github.com>

src/www/assets/404-BsLbhv_m.js

@@ -0,0 +1 @@
+import{t as e}from"./not-found-error-mxpXcTRa.js";var t=e;export{t as component};