feat(session): add per-message created_at timestamps

- Persistence layer (jsonl.go addMsg/SetHistory) normalizes CreatedAt
  when missing so the invariant is guaranteed at the storage boundary
- API layer (session.go) exposes created_at on all transcript message
  types with session.updated fallback for legacy messages
- Frontend uses per-message timestamps when available
- messagesContentEqual ignores CreatedAt for tail-matching after
  JSONL roundtrip

Fixes #2787

web/frontend/src/components/chat/assistant-message.tsx

@@ -117,12 +117,17 @@ export function AssistantMessage({
                   <span className="text-muted-foreground/45">{trimmedModelName}</span>
                 )}
               </div>
-              <IconChevronDown
-                className={cn(
-                  "size-3.5 opacity-0 transition-all duration-200 group-hover:opacity-100",
-                  isExpanded ? "rotate-180" : "",
+              <div className="flex items-center gap-2">
+                {formattedTimestamp && (
+                  <span className="opacity-50">{formattedTimestamp}</span>
                 )}
-              />
+                <IconChevronDown
+                  className={cn(
+                    "size-3.5 opacity-0 transition-all duration-200 group-hover:opacity-100",
+                    isExpanded ? "rotate-180" : "",
+                  )}
+                />
+              </div>
             </div>
           )}
           {(!isCollapsedBlock || isExpanded) && isToolCalls && hasToolCalls && (

web/frontend/src/components/chat/chat-page.tsx

@@ -384,6 +384,7 @@ export function ChatPage() {
                   <UserMessage
                     content={msg.content}
                     attachments={msg.attachments}
+                    timestamp={msg.timestamp}
                   />
                 )}
               </div>

web/frontend/src/components/chat/user-message.tsx

@@ -1,17 +1,23 @@
 import { IconCheck, IconCopy } from "@tabler/icons-react"
+import { useTranslation } from "react-i18next"
 
 import { Button } from "@/components/ui/button"
 import { useCopyToClipboard } from "@/hooks/use-copy-to-clipboard"
+import { formatMessageTime } from "@/hooks/use-pico-chat"
 import { cn } from "@/lib/utils"
 import type { ChatAttachment } from "@/store/chat"
-import { useTranslation } from "react-i18next"
 
 interface UserMessageProps {
   content: string
   attachments?: ChatAttachment[]
+  timestamp?: string | number
 }
 
-export function UserMessage({ content, attachments = [] }: UserMessageProps) {
+export function UserMessage({
+  content,
+  attachments = [],
+  timestamp = "",
+}: UserMessageProps) {
   const { t } = useTranslation()
   const { copy, isCopied } = useCopyToClipboard()
   const hasText = content.trim().length > 0
@@ -22,6 +28,8 @@ export function UserMessage({ content, attachments = [] }: UserMessageProps) {
   const copyMessageLabel = isCopied
     ? t("chat.copiedLabel")
     : t("chat.copyMessage")
+  const formattedTimestamp =
+    timestamp !== "" ? formatMessageTime(timestamp) : ""
 
   return (
     <div className="group flex w-full flex-col items-end gap-1.5">
@@ -81,6 +89,12 @@ export function UserMessage({ content, attachments = [] }: UserMessageProps) {
           </Button>
         </div>
       )}
+
+      {formattedTimestamp && (
+        <span className="px-1 text-[12px] text-zinc-400">
+          {formattedTimestamp}
+        </span>
+      )}
     </div>
   )
 }