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/App.tsx

@@ -1,7 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from "react";
 import type { Channel } from "@/types/Channel";
 import type { Message } from "@/types/Message";
-import { getChannels, getMessages } from "@/api";
+import { getChannels, getMessages, getCurrentUsername } from "@/api";
 import { ChannelSidebar } from "@/components/ChannelSidebar";
 import { MessageItem } from "@/components/MessageItem";
 import { ComposeBox } from "@/components/ComposeBox";