feat: add independent conversation branching

Local conversations can now continue in a new branch after stable content is persisted. Each branch uses new conversation and message IDs, keeps its source visible, and remains independent without copying Tasks, queue items, runs, memories, or artifact ownership.

Conversation Task rows are also denser, align status markers at the right edge, and use an accessible checked success marker without redundant completion text.

Release note: 本地会话现在可从“在新会话中继续”创建独立分支并显示来源；Task 子项布局更紧凑，状态统一显示在右侧。

src/main/ipc.test.ts

@@ -2734,7 +2734,7 @@ describe('registerIpcHandlers local conversation persistence', () => {
     vi.clearAllMocks()
   })
 
-  it('validates and forwards incremental saves and explicit deletions', async () => {
+  it('validates and forwards incremental saves, branches, and explicit deletions', async () => {
     const conversationId =
       '00000000-0000-4000-8000-000000000301'
     const queuedAttachmentId =
@@ -2774,6 +2774,16 @@ describe('registerIpcHandlers local conversation persistence', () => {
         })
       ),
       saveLocalConversations: vi.fn(),
+      branchLocalConversation: vi.fn((input) => ({
+        id: '00000000-0000-4000-8000-000000000305',
+        branch: {
+          sourceConversationId: input.sourceConversationId,
+          sourceTitle: '增量会话'
+        },
+        title: input.title,
+        updatedAt: 2,
+        messages: []
+      })),
       deleteLocalConversation: vi.fn(() => true)
     }
     const contextManager = {
@@ -2838,6 +2848,21 @@ describe('registerIpcHandlers local conversation persistence', () => {
     expect(
       assistantDatabase.saveLocalConversations
     ).toHaveBeenCalledWith(batch)
+    const branchInput = {
+      sourceConversationId: conversationId,
+      title: '增量会话 · 分支'
+    }
+    expect(
+      electronMocks.handlers.get(
+        ipcChannels.conversationsBranchLocal
+      )?.(event, branchInput)
+    ).toMatchObject({
+      branch: { sourceConversationId: conversationId },
+      title: branchInput.title
+    })
+    expect(
+      assistantDatabase.branchLocalConversation
+    ).toHaveBeenCalledWith(branchInput)
     expect(
       electronMocks.handlers.get(
         ipcChannels.conversationsDeleteLocal
@@ -2867,6 +2892,14 @@ describe('registerIpcHandlers local conversation persistence', () => {
         }
       ])
     ).toThrow()
+    expect(() =>
+      electronMocks.handlers.get(
+        ipcChannels.conversationsBranchLocal
+      )?.(event, {
+        sourceConversationId: 'not-a-uuid',
+        title: '无效分支'
+      })
+    ).toThrow()
     expect(() =>
       electronMocks.handlers.get(
         ipcChannels.conversationsDeleteLocal
@@ -3348,6 +3381,17 @@ describe('registerIpcHandlers agent terminal state', () => {
       queueDueSchedules: vi.fn(() => []),
       queueScheduleNow: vi.fn(),
       completeScheduleRun: vi.fn(),
+      branchLocalConversation: vi.fn(() => ({
+        id: '00000000-0000-4000-8000-000000000403',
+        branch: {
+          sourceConversationId:
+            '00000000-0000-4000-8000-000000000402',
+          sourceTitle: '来源会话'
+        },
+        title: '来源会话 · 分支',
+        updatedAt: Date.now(),
+        messages: []
+      })),
       listSchedules: vi.fn(() => []),
       createSchedule: vi.fn(),
       setScheduleEnabled: vi.fn(),
@@ -3471,6 +3515,9 @@ describe('registerIpcHandlers agent terminal state', () => {
       handler: electronMocks.handlers.get(ipcChannels.agentRun),
       statusHandler: electronMocks.handlers.get(ipcChannels.agentStatus),
       cancelHandler: electronMocks.handlers.get(ipcChannels.agentCancel),
+      branchHandler: electronMocks.handlers.get(
+        ipcChannels.conversationsBranchLocal
+      ),
       knowledgeSearchHandler: electronMocks.handlers.get(
         ipcChannels.knowledgeSearch
       ),
@@ -4016,6 +4063,15 @@ describe('registerIpcHandlers agent terminal state', () => {
       })
     ).rejects.toThrow('当前对话已有执行中的请求')
     expect(selectedRuntimes.getRuntime).toHaveBeenCalledOnce()
+    expect(() =>
+      harness.branchHandler?.(trustedEvent(harness.webContents), {
+        sourceConversationId: conversationId,
+        title: '等待 Runtime · 分支'
+      })
+    ).toThrow('当前会话仍有正在执行的请求')
+    expect(
+      harness.assistantDatabase.branchLocalConversation
+    ).not.toHaveBeenCalled()
 
     resolveRuntime?.(runtime)
     await firstRun