feat: restructure frontend routes — vendor/tenant persona split

Splits the flat 3-page UI into /vendor/* (platform:admin) and /tenant/*
(all authenticated users) route trees, with stub pages, new API hooks,
updated Layout with persona-aware sidebar, and SpaController forwarding.

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

src/main/java/net/siegeln/cameleer/saas/config/SpaController.java

@@ -1,13 +1,16 @@
 package net.siegeln.cameleer.saas.config;
 
 import org.springframework.stereotype.Controller;
-import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
 
 @Controller
 public class SpaController {
 
-    @GetMapping(value = {"/", "/login", "/callback", "/environments/**", "/license"})
-    public String spa() {
+    @RequestMapping(value = {
+        "/", "/login", "/callback",
+        "/vendor/**", "/tenant/**"
+    })
+    public String forward() {
         return "forward:/index.html";
     }
 }