Add embedded web chat client with C2S HTTP API

- New DB schema: users, channel_members, messages tables (migration 003)
- Full C2S HTTP API: register, channels, messages, DMs, polling
- Preact SPA embedded via embed.FS, served at GET /
- IRC-style UI: tab bar, channel messages, user list, DM tabs, /commands
- Dark theme, responsive, esbuild-bundled (~19KB)
- Polling-based message delivery (1.5s interval)
- Commands: /join, /part, /msg, /nick

web/embed.go

@@ -0,0 +1,9 @@
+// Package web embeds the built SPA static files.
+package web
+
+import "embed"
+
+// Dist contains the built web client files.
+//
+//go:embed dist/*
+var Dist embed.FS