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/desktop/browser/automation/handler.test.ts

@@ -291,9 +291,11 @@ describe("mountBrowserAutomationHandler", () => {
     if (!workspaceKey) {
       throw new Error("Expected workspace key");
     }
-    const previousFocusedTabId = useWorkspaceLayoutStore
-      .getState()
-      .openTabFocused(workspaceKey, { kind: "draft", draftId: "human-draft" });
+    const previousFocusedTabId = useWorkspaceLayoutStore.getState().openTab({
+      workspaceKey: workspaceKey,
+      target: { kind: "draft", draftId: "human-draft" },
+      intent: "reveal",
+    });
     browser.mount({ serverId: "server-1" });
 
     browser.receive(browserNewTabRequest());

packages/app/src/desktop/browser/automation/handler.ts

@@ -336,9 +336,10 @@ async function openBrowserTabForRequest(params: {
       message: "Cannot create a browser tab without a workspace context.",
     });
   }
-  useWorkspaceLayoutStore.getState().openTabInBackground(workspaceKey, {
-    kind: "browser",
-    browserId,
+  useWorkspaceLayoutStore.getState().openTab({
+    workspaceKey,
+    target: { kind: "browser", browserId },
+    intent: "background",
   });
 
   if (browserHost?.executeAutomationCommand) {

packages/app/src/desktop/browser/new-tab-requests/index.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from "vitest";
 import type { WorkspaceLayout } from "@/stores/workspace-layout-store";
 import { createDefaultLayout } from "@/stores/workspace-layout-store";
-import { openTabInLayoutFocused } from "@/stores/workspace-layout-actions";
+import { FOCUSED_PANE_PLACEMENT, openTabInLayoutFocused } from "@/stores/workspace-layout-actions";
 import { resolveBrowserNewTabRequest, type BrowserNewTabRequest } from ".";
 
 function createLayoutWithBrowser(browserId: string): WorkspaceLayout {
@@ -9,7 +9,8 @@ function createLayoutWithBrowser(browserId: string): WorkspaceLayout {
     layout: createDefaultLayout(),
     target: { kind: "browser", browserId },
     now: 1,
-    explorerPaneId: null,
+    placement: FOCUSED_PANE_PLACEMENT,
+    sidePanelPaneId: null,
   }).layout;
 }
 

packages/app/src/desktop/browser/panel.tsx

@@ -73,6 +73,7 @@ function BrowserPanel() {
 
 export const browserPanelRegistration: PanelRegistration<"browser"> = {
   kind: "browser",
+  resourceKey: (target) => target.browserId,
   component: BrowserPanel,
   useDescriptor: useBrowserPanelDescriptor,
 };