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/pt-BR.ts

@@ -557,7 +557,6 @@ export const ptBR: TranslationResources = {
       loading: "Carregando...",
       modified: "Alterações não salvas",
       loadingAgentTitle: "Carregando título do agente",
-      emptyPane: "Nenhuma aba neste painel.",
       fallback: {
         newAgent: "Novo agente",
         setup: "Configuração",
@@ -599,6 +598,7 @@ export const ptBR: TranslationResources = {
         newBrowser: "Novo navegador",
         maximizePane: "Maximizar painel",
         restorePane: "Restaurar painel",
+        closePane: "Fechar painel",
         exitFocusMode: "Sair do modo de foco",
         splitRight: "Dividir painel à direita",
         splitDown: "Dividir painel abaixo",
@@ -608,10 +608,10 @@ export const ptBR: TranslationResources = {
         terminalProfilesMenu: "Perfis de terminal",
         editTerminalProfiles: "Editar perfis…",
       },
-      explorer: {
-        open: "Abrir explorador",
-        close: "Fechar explorador",
-        toggle: "Alternar explorador",
+      sidePanel: {
+        open: "Abrir painel lateral",
+        close: "Fechar painel lateral",
+        toggle: "Alternar painel lateral",
         changes: "Alterações",
         files: "Arquivos",
       },
@@ -713,6 +713,10 @@ export const ptBR: TranslationResources = {
         stopFailed: "Falha ao parar {{scriptName}}",
       },
     },
+    tree: {
+      showFolderTree: "Mostrar árvore de pastas",
+      hideFolderTree: "Ocultar árvore de pastas",
+    },
     git: {
       actions: {
         moreOptions: "Mais opções",
@@ -877,8 +881,6 @@ export const ptBR: TranslationResources = {
         split: "Diff lado a lado",
         switchToUnified: "Alternar para diff unificado",
         switchToSplit: "Alternar para diff lado a lado",
-        showTreeView: "Mostrar árvore de pastas",
-        hideTreeView: "Ocultar árvore de pastas",
         options: "Opções do diff",
         hideWhitespace: "Ocultar espaços em branco",
         showWhitespace: "Mostrar espaços em branco",
@@ -1950,6 +1952,11 @@ export const ptBR: TranslationResources = {
         description: "Linhas mantidas no buffer do terminal integrado",
         accessibilityLabel: "Linhas do scrollback do terminal",
       },
+      sidePanelRouting: {
+        label: "Abrir abas de apoio no painel lateral",
+        description:
+          "Links de arquivo, pull requests e progresso da configuração abrem ao lado do seu trabalho em vez do painel em foco",
+      },
       autoExpandReasoning: {
         label: "Sempre expandir raciocínio",
         description:
@@ -2134,11 +2141,11 @@ export const ptBR: TranslationResources = {
         toggleCommandCenter: "Alternar central de comandos",
         showKeyboardShortcuts: "Mostrar atalhos de teclado",
         toggleLeftSidebar: "Alternar barra lateral esquerda",
-        toggleRightSidebar: "Alternar barra lateral direita",
+        toggleRightSidebar: "Alternar painel lateral",
         toggleBothSidebars: "Alternar ambas as barras laterais",
         toggleSettings: "Alternar Configurações",
         toggleFocusMode: "Alternar modo de foco",
-        toggleExplorerPaneMaximization: "Alternar maximização do painel Explorador",
+        toggleExplorerPaneMaximization: "Alternar maximização do painel lateral",
         cycleTheme: "Alternar tema",
         focusMessageInput: "Focar entrada de mensagem",
         cycleAgentMode: "Alternar modo do agente",