feat: show full date/time and unix timestamp on time hover

Hovering over the message time now shows a tooltip with the full
human-readable date/time and the unix timestamp below it.

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

client/src/components/message-bubble.riot

@@ -22,7 +22,7 @@
     <div class="message-body">
       <div class={'message-header ' + (props.isOwn ? 'own' : '')}>
         <span class="sender-name">{props.message?.sender_name}</span>
-        <span class="message-time">{formatTime(props.message?.created_at)}</span>
+        <span class="message-time" title={fullTimestamp(props.message?.created_at)}>{formatTime(props.message?.created_at)}</span>
         <span if={props.message?.hash} class="message-hash" title={props.message.hash}>{props.message.hash.slice(0, 7)}</span>
       </div>
       <div if={hasToolResults()} class="tool-results-section">
@@ -431,6 +431,14 @@
         return date.toLocaleTimeString([], { hour: '2-digit', minute: '2-digit' })
       },
 
+      fullTimestamp(dateStr) {
+        if (!dateStr) return ''
+        const date = new Date(dateStr)
+        const human = date.toLocaleString([], { dateStyle: 'full', timeStyle: 'long' })
+        const unix = Math.floor(date.getTime() / 1000)
+        return `${human}\nUnix: ${unix}`
+      },
+
       formatModel(model) {
         if (!model) return 'unknown'
         // "openai/gpt-4o" → "gpt-4o", "anthropic/claude-3.5-sonnet" → "claude-3.5-sonnet"