feat: MVP two-user chat via embedded SPA (#9)

Backend:
- Session/client UUID model: sessions table (uuid, nick, signing_key),
  clients table (uuid, session_id, token) with per-client message queues
- MOTD delivery as IRC numeric messages (375/372/376) on connect
- EnqueueToSession fans out to all clients of a session
- EnqueueToClient for targeted delivery (MOTD)
- All queries updated for session/client model

SPA client:
- Long-poll loop (15s timeout) instead of setInterval
- IRC message envelope parsing (command/from/to/body)
- Display JOIN/PART/NICK/TOPIC/QUIT system messages
- Nick change via /nick command
- Topic display in header bar
- Unread count badges on inactive tabs
- Auto-rejoin channels on reconnect (localStorage)
- Connection status indicator
- Message deduplication by UUID
- Channel history loaded on join
- /topic command support

Closes #9

internal/db/schema/001_initial.sql

@@ -1,15 +1,28 @@
 -- Chat server schema (pre-1.0 consolidated)
 PRAGMA foreign_keys = ON;
 
--- Users: IRC-style sessions (no passwords, just nick + token)
-CREATE TABLE IF NOT EXISTS users (
+-- Sessions: IRC-style sessions (no passwords, nick + optional signing key)
+CREATE TABLE IF NOT EXISTS sessions (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
+    uuid TEXT NOT NULL UNIQUE,
     nick TEXT NOT NULL UNIQUE,
+    signing_key TEXT NOT NULL DEFAULT '',
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    last_seen DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_sessions_uuid ON sessions(uuid);
+
+-- Clients: each session can have multiple connected clients
+CREATE TABLE IF NOT EXISTS clients (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    uuid TEXT NOT NULL UNIQUE,
+    session_id INTEGER NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
     token TEXT NOT NULL UNIQUE,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
     last_seen DATETIME DEFAULT CURRENT_TIMESTAMP
 );
-CREATE INDEX IF NOT EXISTS idx_users_token ON users(token);
+CREATE INDEX IF NOT EXISTS idx_clients_token ON clients(token);
+CREATE INDEX IF NOT EXISTS idx_clients_session ON clients(session_id);
 
 -- Channels
 CREATE TABLE IF NOT EXISTS channels (
@@ -24,9 +37,9 @@ CREATE TABLE IF NOT EXISTS channels (
 CREATE TABLE IF NOT EXISTS channel_members (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     channel_id INTEGER NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
-    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    session_id INTEGER NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
     joined_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-    UNIQUE(channel_id, user_id)
+    UNIQUE(channel_id, session_id)
 );
 
 -- Messages: IRC envelope format
@@ -46,9 +59,9 @@ CREATE INDEX IF NOT EXISTS idx_messages_created ON messages(created_at);
 -- Per-client message queues for fan-out delivery
 CREATE TABLE IF NOT EXISTS client_queues (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
-    user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    client_id INTEGER NOT NULL REFERENCES clients(id) ON DELETE CASCADE,
     message_id INTEGER NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
-    UNIQUE(user_id, message_id)
+    UNIQUE(client_id, message_id)
 );
-CREATE INDEX IF NOT EXISTS idx_client_queues_user ON client_queues(user_id, id);
+CREATE INDEX IF NOT EXISTS idx_client_queues_client ON client_queues(client_id, id);