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

web/dist/style.css

@@ -14,6 +14,9 @@
   --tab-active: #e94560;
   --tab-bg: #16213e;
   --tab-hover: #1a1a3e;
+  --topic-bg: #121a30;
+  --unread-bg: #e94560;
+  --warn: #f0ad4e;
 }
 
 html, body, #root {
@@ -86,6 +89,7 @@ html, body, #root {
   border-bottom: 1px solid var(--border);
   overflow-x: auto;
   flex-shrink: 0;
+  align-items: center;
 }
 
 .tab {
@@ -95,6 +99,7 @@ html, body, #root {
   white-space: nowrap;
   color: var(--text-muted);
   user-select: none;
+  position: relative;
 }
 
 .tab:hover {
@@ -116,6 +121,43 @@ html, body, #root {
   color: var(--accent);
 }
 
+.tab .unread-badge {
+  display: inline-block;
+  background: var(--unread-bg);
+  color: white;
+  font-size: 10px;
+  font-weight: bold;
+  padding: 1px 5px;
+  border-radius: 8px;
+  margin-left: 6px;
+  min-width: 16px;
+  text-align: center;
+}
+
+/* Connection status */
+.connection-status {
+  padding: 4px 12px;
+  background: var(--warn);
+  color: #1a1a2e;
+  font-size: 12px;
+  font-weight: bold;
+  white-space: nowrap;
+  flex-shrink: 0;
+}
+
+/* Topic bar */
+.topic-bar {
+  padding: 6px 12px;
+  background: var(--topic-bg);
+  border-bottom: 1px solid var(--border);
+  color: var(--text-muted);
+  font-size: 12px;
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  flex-shrink: 0;
+}
+
 /* Content area */
 .content {
   display: flex;
@@ -243,6 +285,7 @@ html, body, #root {
   gap: 8px;
   background: var(--bg-secondary);
   border-bottom: 1px solid var(--border);
+  margin-left: auto;
 }
 
 .join-dialog input {