fix: clarify conversation list timestamps

The recent-conversation list previously showed only a time, making older entries ambiguous. It now shows local time for conversations updated today, a local date for older entries, and includes the year when needed; the full timestamp remains available on hover.

Release note: 会话列表现在当天显示时间、较早会话显示日期，跨年记录同时显示年份，更容易快速判断最近更新时间。

UI-DESIGN.md

@@ -490,6 +490,7 @@ GoodBuddy 是可调整窗口大小的桌面应用。响应式设计优先保证
 - 使用 `standard` 壳层和统一 `PageHeader`。
 - 搜索、范围和时间筛选位于筛选工具栏。
 - 行项目统一显示标题、范围、最近更新时间和必要状态。
+- 侧栏最近会话的更新时间在当天显示本地时间，非当天显示本地日期；非当年记录必须同时显示年份。悬停时间信息时提供完整日期和时间。
 - 删除入口使用 `danger-ghost`，并按数据可恢复性执行确认或撤销策略。
 
 ### 13.3 知识库

src/renderer/src/App.tsx

@@ -181,7 +181,11 @@ import type { ReleaseNotesSnapshot } from '../../shared/release-notes-contracts'
 import { ReleaseNotesDialog } from './ReleaseNotesDialog'
 import { scheduleIdleRoutePreload } from './idle-route-preload'
 import { createPreloadableComponent } from './preloadable-component'
-import { formatTime, type TimeFormatLocale } from './time-format'
+import {
+  formatConversationListTime,
+  type TimeFormatLocale
+} from './time-format'
+import { formatMediumDateTime } from './locale-formatters'
 import { formatCompactTokens } from './token-format'
 import {
   pruneKeepAliveEntries,
@@ -6464,7 +6468,22 @@ function App(): React.JSX.Element {
                       />
                     )}
                   </span>
-                  <small>{formatTime(conversation.updatedAt, locale)}</small>
+                  <small>
+                    <time
+                      dateTime={new Date(
+                        conversation.updatedAt
+                      ).toISOString()}
+                      title={formatMediumDateTime(
+                        conversation.updatedAt,
+                        locale
+                      )}
+                    >
+                      {formatConversationListTime(
+                        conversation.updatedAt,
+                        locale
+                      )}
+                    </time>
+                  </small>
                 </button>
                 {activeConversationIds.has(conversation.id) && (
                   <span

src/renderer/src/time-format.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
-import { formatTime } from './time-format'
+import { formatConversationListTime, formatTime } from './time-format'
 
 describe('formatTime', () => {
   afterEach(() => {
@@ -27,4 +27,47 @@ describe('formatTime', () => {
     expect(secondChinese).not.toBe('')
     expect(formatter).toHaveBeenCalledTimes(2)
   })
+
+  it('shows the time for a conversation updated today', () => {
+    const timestamp = new Date(2026, 7, 17, 9, 5).getTime()
+    const referenceTimestamp = new Date(2026, 7, 17, 23, 30).getTime()
+
+    expect(
+      formatConversationListTime(timestamp, 'zh-CN', referenceTimestamp)
+    ).toBe(
+      new Intl.DateTimeFormat('zh-CN', {
+        hour: '2-digit',
+        minute: '2-digit'
+      }).format(timestamp)
+    )
+  })
+
+  it('shows the date after the local calendar day changes', () => {
+    const timestamp = new Date(2026, 7, 16, 23, 30).getTime()
+    const referenceTimestamp = new Date(2026, 7, 17, 0, 15).getTime()
+
+    expect(
+      formatConversationListTime(timestamp, 'zh-CN', referenceTimestamp)
+    ).toBe(
+      new Intl.DateTimeFormat('zh-CN', {
+        month: 'short',
+        day: 'numeric'
+      }).format(timestamp)
+    )
+  })
+
+  it('includes the year for a conversation from another year', () => {
+    const timestamp = new Date(2025, 11, 31, 23, 30).getTime()
+    const referenceTimestamp = new Date(2026, 0, 1, 0, 15).getTime()
+
+    expect(
+      formatConversationListTime(timestamp, 'en-US', referenceTimestamp)
+    ).toBe(
+      new Intl.DateTimeFormat('en-US', {
+        year: 'numeric',
+        month: 'short',
+        day: 'numeric'
+      }).format(timestamp)
+    )
+  })
 })

src/renderer/src/time-format.ts

@@ -4,6 +4,14 @@ const timeFormatters: Partial<
   Record<TimeFormatLocale, Intl.DateTimeFormat>
 > = {}
 
+const conversationDateFormatters: Partial<
+  Record<TimeFormatLocale, Intl.DateTimeFormat>
+> = {}
+
+const conversationDateWithYearFormatters: Partial<
+  Record<TimeFormatLocale, Intl.DateTimeFormat>
+> = {}
+
 export function formatTime(
   timestamp: number,
   locale: TimeFormatLocale
@@ -16,3 +24,43 @@ export function formatTime(
     }))
   return formatter.format(timestamp)
 }
+
+export function formatConversationListTime(
+  timestamp: number,
+  locale: TimeFormatLocale,
+  referenceTimestamp = Date.now()
+): string {
+  const date = new Date(timestamp)
+  const referenceDate = new Date(referenceTimestamp)
+  const isSameYear = date.getFullYear() === referenceDate.getFullYear()
+  const isToday =
+    isSameYear &&
+    date.getMonth() === referenceDate.getMonth() &&
+    date.getDate() === referenceDate.getDate()
+
+  if (isToday) {
+    return formatTime(timestamp, locale)
+  }
+
+  if (isSameYear) {
+    const formatter =
+      conversationDateFormatters[locale] ??
+      (conversationDateFormatters[locale] = new Intl.DateTimeFormat(locale, {
+        month: 'short',
+        day: 'numeric'
+      }))
+    return formatter.format(timestamp)
+  }
+
+  const formatter =
+    conversationDateWithYearFormatters[locale] ??
+    (conversationDateWithYearFormatters[locale] = new Intl.DateTimeFormat(
+      locale,
+      {
+        year: 'numeric',
+        month: 'short',
+        day: 'numeric'
+      }
+    ))
+  return formatter.format(timestamp)
+}