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/ja.ts

@@ -557,7 +557,6 @@ export const ja: TranslationResources = {
       loading: "読み込み中...",
       modified: "未保存の変更",
       loadingAgentTitle: "エージェントタイトルを読み込み中",
-      emptyPane: "このペインにタブがありません。",
       fallback: {
         newAgent: "新しいエージェント",
         setup: "セットアップ",
@@ -600,6 +599,7 @@ export const ja: TranslationResources = {
         newBrowser: "新しいブラウザ",
         maximizePane: "ペインを最大化",
         restorePane: "ペインを元に戻す",
+        closePane: "ペインを閉じる",
         exitFocusMode: "フォーカスモードを終了",
         splitRight: "右にペインを分割",
         splitDown: "下にペインを分割",
@@ -609,10 +609,10 @@ export const ja: TranslationResources = {
         terminalProfilesMenu: "ターミナルプロファイル",
         editTerminalProfiles: "プロファイルを編集…",
       },
-      explorer: {
-        open: "エクスプローラーを開く",
-        close: "エクスプローラーを閉じる",
-        toggle: "エクスプローラーを切り替え",
+      sidePanel: {
+        open: "サイドパネルを開く",
+        close: "サイドパネルを閉じる",
+        toggle: "サイドパネルを切り替え",
         changes: "変更",
         files: "ファイル",
       },
@@ -712,6 +712,10 @@ export const ja: TranslationResources = {
         stopFailed: "{{scriptName}}の停止に失敗しました",
       },
     },
+    tree: {
+      showFolderTree: "フォルダツリーを表示",
+      hideFolderTree: "フォルダツリーを非表示",
+    },
     git: {
       actions: {
         moreOptions: "その他のオプション",
@@ -866,8 +870,6 @@ export const ja: TranslationResources = {
         split: "左右比較",
         switchToUnified: "ユニファイド差分に切り替え",
         switchToSplit: "左右比較差分に切り替え",
-        showTreeView: "フォルダツリーを表示",
-        hideTreeView: "フォルダツリーを非表示",
         options: "差分オプション",
         hideWhitespace: "空白を非表示",
         showWhitespace: "空白を表示",
@@ -1934,6 +1936,11 @@ export const ja: TranslationResources = {
         description: "組み込みターミナルバッファに保持する行数",
         accessibilityLabel: "ターミナルスクロールバック行数",
       },
+      sidePanelRouting: {
+        label: "補助タブをサイドパネルで開く",
+        description:
+          "ファイルリンク、プルリクエスト、セットアップの進行状況が、フォーカス中のペインではなく作業の横に開きます",
+      },
       autoExpandReasoning: {
         label: "常に思考プロセスを展開",
         description: "デフォルトでAIのエージェント思考・推論ブロックを完全に展開して表示します",
@@ -2118,11 +2125,11 @@ export const ja: TranslationResources = {
         toggleCommandCenter: "コマンドセンターを切り替え",
         showKeyboardShortcuts: "キーボードショートカットを表示",
         toggleLeftSidebar: "左サイドバーを切り替え",
-        toggleRightSidebar: "右サイドバーを切り替え",
+        toggleRightSidebar: "サイドパネルを切り替え",
         toggleBothSidebars: "両方のサイドバーを切り替え",
         toggleSettings: "設定を切り替え",
         toggleFocusMode: "フォーカスモードを切り替え",
-        toggleExplorerPaneMaximization: "エクスプローラーペインの最大化を切り替え",
+        toggleExplorerPaneMaximization: "サイドパネルの最大化を切り替え",
         cycleTheme: "テーマを順に切り替え",
         focusMessageInput: "メッセージ入力にフォーカス",
         cycleAgentMode: "エージェントモードを順に切り替え",