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/hooks/use-settings/storage.ts

@@ -77,6 +77,8 @@ export interface AppSettings {
   toolCallDetailLevel: ToolCallDetailLevel;
   chatOutlineEnabled: boolean;
   vimKeybindings: boolean;
+  /** Route implicitly opened supporting tabs into the Side panel. Desktop only. */
+  openSupportingTabsInSidePanel: boolean;
 }
 
 export interface Settings extends AppSettings {
@@ -124,6 +126,7 @@ const StoredAppSettingsSchema = z.strictObject({
   compactToolCalls: z.boolean().optional(),
   chatOutlineEnabled: z.boolean().optional(),
   vimKeybindings: z.boolean().optional(),
+  openSupportingTabsInSidePanel: z.boolean().optional(),
   // COMPAT(rendererDesktopSettings): these fields used to share this renderer-owned key.
   manageBuiltInDaemon: z.boolean().optional(),
   releaseChannel: z.enum(["stable", "beta"]).optional(),
@@ -153,6 +156,7 @@ export const DEFAULT_CLIENT_SETTINGS: AppSettings = {
   toolCallDetailLevel: "detailed",
   chatOutlineEnabled: true,
   vimKeybindings: false,
+  openSupportingTabsInSidePanel: true,
 };
 
 export const DEFAULT_APP_SETTINGS: Settings = {
@@ -315,6 +319,9 @@ function pickBooleanAppSettings(stored: StoredAppSettings): Partial<AppSettings>
   if (typeof stored.chatOutlineEnabled === "boolean") {
     result.chatOutlineEnabled = stored.chatOutlineEnabled;
   }
+  if (typeof stored.openSupportingTabsInSidePanel === "boolean") {
+    result.openSupportingTabsInSidePanel = stored.openSupportingTabsInSidePanel;
+  }
   return result;
 }