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/ar.ts

@@ -553,7 +553,6 @@ export const ar: TranslationResources = {
       loading: "تحميل...",
       modified: "تغييرات غير محفوظة",
       loadingAgentTitle: "جارٍ تحميل عنوان الوكيل",
-      emptyPane: "لا توجد علامات تبويب في هذا الجزء.",
       fallback: {
         newAgent: "وكيل جديد",
         setup: "يثبت",
@@ -595,6 +594,7 @@ export const ar: TranslationResources = {
         newBrowser: "متصفح جديد",
         maximizePane: "تكبير الجزء",
         restorePane: "استعادة الجزء",
+        closePane: "إغلاق الجزء",
         exitFocusMode: "إنهاء وضع التركيز",
         splitRight: "تقسيم الجزء الأيمن",
         splitDown: "تقسيم الجزء لأسفل",
@@ -604,10 +604,10 @@ export const ar: TranslationResources = {
         terminalProfilesMenu: "Terminal profiles",
         editTerminalProfiles: "Edit profiles…",
       },
-      explorer: {
-        open: "افتح المستكشف",
-        close: "إغلاق المستكشف",
-        toggle: "تبديل المستكشف",
+      sidePanel: {
+        open: "افتح اللوحة الجانبية",
+        close: "إغلاق اللوحة الجانبية",
+        toggle: "تبديل اللوحة الجانبية",
         changes: "التغييرات",
         files: "ملفات",
       },
@@ -710,6 +710,10 @@ export const ar: TranslationResources = {
         stopFailed: "فشل إيقاف{{scriptName}}",
       },
     },
+    tree: {
+      showFolderTree: "إظهار شجرة المجلدات",
+      hideFolderTree: "إخفاء شجرة المجلدات",
+    },
     git: {
       actions: {
         moreOptions: "المزيد من الخيارات",
@@ -855,8 +859,6 @@ export const ar: TranslationResources = {
         split: "فرق جنبًا إلى جنب",
         switchToUnified: "التبديل إلى الفرق الموحد",
         switchToSplit: "التبديل إلى الفرق جنبًا إلى جنب",
-        showTreeView: "إظهار شجرة المجلدات",
-        hideTreeView: "إخفاء شجرة المجلدات",
         options: "خيارات الفرق",
         hideWhitespace: "إخفاء المسافة البيضاء",
         showWhitespace: "إظهار المسافة البيضاء",
@@ -1917,6 +1919,11 @@ export const ar: TranslationResources = {
         description: "يتم الاحتفاظ بالخطوط في المخزن المؤقت الطرفي المدمج",
         accessibilityLabel: "خطوط التمرير Terminal",
       },
+      sidePanelRouting: {
+        label: "فتح علامات التبويب المساعدة في اللوحة الجانبية",
+        description:
+          "تفتح روابط الملفات وطلبات السحب وتقدم الإعداد بجوار عملك بدلاً من الجزء المركّز عليه",
+      },
       autoExpandReasoning: {
         label: "عرض التفكير دائماً",
         description: "إظهار تفكير الوكيل وخطوات الاستدلال بشكل كامل بشكل افتراضي",
@@ -2100,11 +2107,11 @@ export const ar: TranslationResources = {
         toggleCommandCenter: "تبديل مركز القيادة",
         showKeyboardShortcuts: "إظهار اختصارات لوحة المفاتيح",
         toggleLeftSidebar: "تبديل الشريط الجانبي الأيسر",
-        toggleRightSidebar: "تبديل الشريط الجانبي الأيمن",
+        toggleRightSidebar: "تبديل اللوحة الجانبية",
         toggleBothSidebars: "تبديل كلا الشريطين الجانبيين",
         toggleSettings: "تبديل الإعدادات",
         toggleFocusMode: "تبديل وضع التركيز",
-        toggleExplorerPaneMaximization: "تبديل تكبير جزء المستكشف",
+        toggleExplorerPaneMaximization: "تبديل تكبير اللوحة الجانبية",
         cycleTheme: "موضوع الدورة",
         focusMessageInput: "التركيز على إدخال الرسالة",
         cycleAgentMode: "تبديل وضع الوكيل",