S3+S4: user param auth, static file serving, Docker deploy config

- replace hardcoded benji with ?user= query param
- add GET /api/users and GET /api/me?user= endpoints
- serve frontend static files via tower-http ServeDir
- add multi-stage Dockerfile (Rust + Vite → single image)
- add docker-compose.yml + Caddyfile for apes.unslope.com
- frontend: getCurrentUsername() from URL param → localStorage
- sidebar shows current user

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/colony/src/components/ChannelSidebar.tsx

@@ -1,6 +1,6 @@
 import { useState } from "react";
 import type { Channel } from "@/types/Channel";
-import { createChannel } from "@/api";
+import { createChannel, getCurrentUsername } from "@/api";
 
 interface Props {
   channels: Channel[];
@@ -61,6 +61,12 @@ export function ChannelSidebar({
         ))}
       </div>
 
+      {/* Current user */}
+      <div className="px-3 py-2 border-t border-sidebar-border text-[11px]">
+        <span className="text-muted-foreground">logged in as </span>
+        <span className="font-bold text-foreground">{getCurrentUsername()}</span>
+      </div>
+
       {/* New channel input */}
       <div className="p-2 border-t border-sidebar-border">
         <input