feat(app): make the side panel user-directed (#3605)

* feat(app): let the file panel close its tree rail

The file panel's tree rail was permanent on desktop — the changes panel
had a folder-tree toggle in its toolbar, the file toolbar had nothing, so
the rail was there whether you wanted it or not.

The changes panel's private DesktopTreeToggle becomes a shared
TreeRailToggle so both toolbars render the same affordance; that also
drops the treeToggleStyle prop threaded through ChangesToolbarProps,
which recomputed exactly what the component already built internally.
The i18n keys move out of workspace.git.diff into workspace.tree, since a
shared component reaching into the git namespace would rot.

The two rails keep independent flags — the file rail in panel-store, the
changes rail in useChangesPreferences — so closing one leaves the other
alone. FileTreeToggle wires itself to the store rather than taking a
callback: the bar renders several components below FilePanel, where the
rail actually lives.

Both rails also get narrower (320 -> 260 default, 200 -> 180 minimum).
The persist version bump resets treeRailWidth on purpose: nearly every
existing install carries the old 320 default, so without the reset the
narrower rail would only ever reach new users. Anyone who dragged their
rail to a deliberate custom width loses it once.

Note that MIN_CONTENT_WIDTH still clamps the rail to
`containerWidth - 240`, so in a narrow pane the new default never
applies — that floor, not the default, is what sets the width there.

* feat(app): make the side panel an empty workspace surface

The fixed, hideable side panel keeps supporting context beside an agent without seeding content. Pane-local choices stay local, while implicit supporting opens follow a persisted routing preference.

* test(app): exercise side panel policy through its capability boundary

The browser rail journey already covers the real tree-toggle behavior. Side-panel policy tests now pass their platform capability through the public boundary instead of replacing a production module.

* test(app): expect an empty side panel after upgrade

* test(app): expect an empty first split pane

* test(app): account for the empty side panel in split retention

* feat(app): make workspace panel tabs independent

Give explicit New actions fresh tab instances, keep panel state opaque to tab management, preserve File navigation context, and stabilize uncached File transitions.

* fix(app): preserve file conflicts during preview refresh

Keep the current preview mounted for same-target reads and conversions so the editor retains its conflict observation state. Clear retained previews only when the file target changes.

packages/app/src/i18n/resources/zh-CN.ts

@@ -553,7 +553,6 @@ export const zhCN: TranslationResources = {
       loading: "正在加载...",
       modified: "未保存的更改",
       loadingAgentTitle: "正在加载 Agent 标题",
-      emptyPane: "此窗格中没有标签。",
       fallback: {
         newAgent: "新建 Agent",
         setup: "Setup",
@@ -595,6 +594,7 @@ export const zhCN: TranslationResources = {
         newBrowser: "新建浏览器",
         maximizePane: "最大化窗格",
         restorePane: "还原窗格",
+        closePane: "关闭窗格",
         exitFocusMode: "退出专注模式",
         splitRight: "向右拆分窗格",
         splitDown: "向下拆分窗格",
@@ -604,10 +604,10 @@ export const zhCN: TranslationResources = {
         terminalProfilesMenu: "Terminal profiles",
         editTerminalProfiles: "Edit profiles…",
       },
-      explorer: {
-        open: "打开 explorer",
-        close: "关闭 explorer",
-        toggle: "切换 explorer",
+      sidePanel: {
+        open: "打开侧边面板",
+        close: "关闭侧边面板",
+        toggle: "切换侧边面板",
         changes: "变更",
         files: "文件",
       },
@@ -706,6 +706,10 @@ export const zhCN: TranslationResources = {
         stopFailed: "停止 {{scriptName}} 失败",
       },
     },
+    tree: {
+      showFolderTree: "显示文件夹树",
+      hideFolderTree: "隐藏文件夹树",
+    },
     git: {
       actions: {
         moreOptions: "更多选项",
@@ -847,8 +851,6 @@ export const zhCN: TranslationResources = {
         split: "Side-by-side diff",
         switchToUnified: "切换到统一差异视图",
         switchToSplit: "切换到并排差异视图",
-        showTreeView: "显示文件夹树",
-        hideTreeView: "隐藏文件夹树",
         options: "Diff 选项",
         hideWhitespace: "隐藏空白差异",
         showWhitespace: "显示空白差异",
@@ -1894,6 +1896,10 @@ export const zhCN: TranslationResources = {
         description: "内置终端缓冲区保留的行数",
         accessibilityLabel: "终端回滚行数",
       },
+      sidePanelRouting: {
+        label: "在侧边面板中打开辅助标签页",
+        description: "文件链接、拉取请求和设置进度会在你的工作旁边打开，而不是在聚焦的窗格中",
+      },
       autoExpandReasoning: {
         label: "始终展开推理过程",
         description: "默认情况下完全展开 AI 的思考和推理过程",
@@ -2076,11 +2082,11 @@ export const zhCN: TranslationResources = {
         toggleCommandCenter: "切换命令中心",
         showKeyboardShortcuts: "显示键盘快捷键",
         toggleLeftSidebar: "切换左侧边栏",
-        toggleRightSidebar: "切换右侧边栏",
+        toggleRightSidebar: "切换侧边面板",
         toggleBothSidebars: "切换两侧边栏",
         toggleSettings: "切换设置",
         toggleFocusMode: "切换专注模式",
-        toggleExplorerPaneMaximization: "切换资源管理器窗格最大化",
+        toggleExplorerPaneMaximization: "切换侧边面板最大化",
         cycleTheme: "循环切换主题",
         focusMessageInput: "聚焦消息输入框",
         cycleAgentMode: "循环切换代理模式",