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/es.ts

@@ -557,7 +557,6 @@ export const es: TranslationResources = {
       loading: "Cargando...",
       modified: "Cambios sin guardar",
       loadingAgentTitle: "Título del agente de carga",
-      emptyPane: "No hay pestañas en este panel.",
       fallback: {
         newAgent: "Nuevo agente",
         setup: "Configuración",
@@ -600,6 +599,7 @@ export const es: TranslationResources = {
         newBrowser: "Nuevo navegador",
         maximizePane: "Maximizar panel",
         restorePane: "Restaurar panel",
+        closePane: "Cerrar panel",
         exitFocusMode: "Salir del modo de concentración",
         splitRight: "Panel dividido a la derecha",
         splitDown: "Dividir panel hacia abajo",
@@ -609,10 +609,10 @@ export const es: TranslationResources = {
         terminalProfilesMenu: "Terminal profiles",
         editTerminalProfiles: "Edit profiles…",
       },
-      explorer: {
-        open: "Explorador abierto",
-        close: "Cerrar explorador",
-        toggle: "Alternar explorador",
+      sidePanel: {
+        open: "Abrir panel lateral",
+        close: "Cerrar panel lateral",
+        toggle: "Alternar panel lateral",
         changes: "Cambios",
         files: "Archivos",
       },
@@ -716,6 +716,10 @@ export const es: TranslationResources = {
         stopFailed: "No se pudo detener{{scriptName}}",
       },
     },
+    tree: {
+      showFolderTree: "Mostrar árbol de carpetas",
+      hideFolderTree: "Ocultar árbol de carpetas",
+    },
     git: {
       actions: {
         moreOptions: "Más opciones",
@@ -886,8 +890,6 @@ export const es: TranslationResources = {
         split: "Diferencia de lado a lado",
         switchToUnified: "Cambiar a diferencia unificada",
         switchToSplit: "Cambiar a diferencia lado a lado",
-        showTreeView: "Mostrar árbol de carpetas",
-        hideTreeView: "Ocultar árbol de carpetas",
         options: "Opciones de diferencia",
         hideWhitespace: "Ocultar espacios en blanco",
         showWhitespace: "Mostrar espacios en blanco",
@@ -1966,6 +1968,11 @@ export const es: TranslationResources = {
         description: "Líneas mantenidas en el búfer de terminal incorporado",
         accessibilityLabel: "Líneas del historial de terminal",
       },
+      sidePanelRouting: {
+        label: "Abrir pestañas de apoyo en el panel lateral",
+        description:
+          "Los enlaces a archivos, las solicitudes de incorporación y el progreso de configuración se abren junto a tu trabajo en lugar del panel enfocado",
+      },
       autoExpandReasoning: {
         label: "Siempre expandir razonamiento",
         description:
@@ -2151,11 +2158,11 @@ export const es: TranslationResources = {
         toggleCommandCenter: "Alternar centro de comando",
         showKeyboardShortcuts: "Mostrar atajos de teclado",
         toggleLeftSidebar: "Alternar barra lateral izquierda",
-        toggleRightSidebar: "Alternar barra lateral derecha",
+        toggleRightSidebar: "Alternar panel lateral",
         toggleBothSidebars: "Alternar ambas barras laterales",
         toggleSettings: "Alternar configuración",
         toggleFocusMode: "Alternar modo de enfoque",
-        toggleExplorerPaneMaximization: "Alternar maximización del panel Explorador",
+        toggleExplorerPaneMaximization: "Alternar maximización del panel lateral",
         cycleTheme: "Tema del ciclo",
         focusMessageInput: "Entrada de mensaje de enfoque",
         cycleAgentMode: "Alternar modo del agente",