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/screens/settings-screen.tsx

@@ -77,6 +77,7 @@ import { KeyboardShortcutsSection } from "@/screens/settings/keyboard-shortcuts-
 import { EditorSection } from "@/screens/settings/editor-section";
 import { Button } from "@/components/ui/button";
 import { Switch } from "@/components/ui/switch";
+import { supportsDesktopPaneSplits } from "@/constants/layout";
 import { CommunityLinks } from "@/components/community-links";
 import { SegmentedControl } from "@/components/ui/segmented-control";
 import { DropdownMenu, DropdownMenuContent, DropdownMenuItem } from "@/components/ui/dropdown-menu";
@@ -273,6 +274,7 @@ interface GeneralSectionProps {
   handleServiceUrlBehaviorChange: (behavior: ServiceUrlBehavior) => void;
   handleLanguageChange: (language: AppLanguage) => void;
   handleTerminalScrollbackLinesChange: (lines: number) => void;
+  handleSidePanelRoutingChange: (enabled: boolean) => void;
 }
 
 interface ServiceUrlBehaviorMenuItemProps {
@@ -329,6 +331,7 @@ function GeneralSection({
   handleServiceUrlBehaviorChange,
   handleLanguageChange,
   handleTerminalScrollbackLinesChange,
+  handleSidePanelRoutingChange,
 }: GeneralSectionProps) {
   const { t, i18n } = useTranslation();
   const activeLocale = getActiveLocale(i18n.language);
@@ -459,6 +462,23 @@ function GeneralSection({
             accessibilityLabel={t("settings.general.terminalScrollback.accessibilityLabel")}
           />
         </View>
+        {supportsDesktopPaneSplits() ? (
+          <View style={[settingsStyles.row, settingsStyles.rowBorder]}>
+            <View style={settingsStyles.rowContent}>
+              <Text style={settingsStyles.rowTitle}>
+                {t("settings.general.sidePanelRouting.label")}
+              </Text>
+              <Text style={settingsStyles.rowHint}>
+                {t("settings.general.sidePanelRouting.description")}
+              </Text>
+            </View>
+            <Switch
+              value={settings.openSupportingTabsInSidePanel}
+              onValueChange={handleSidePanelRoutingChange}
+              accessibilityLabel={t("settings.general.sidePanelRouting.label")}
+            />
+          </View>
+        ) : null}
       </View>
     </SettingsSection>
   );
@@ -1211,6 +1231,13 @@ export default function SettingsScreen({ view, openAddHostIntent = null }: Setti
     [updateSettings],
   );
 
+  const handleSidePanelRoutingChange = useCallback(
+    (openSupportingTabsInSidePanel: boolean) => {
+      void updateSettings({ openSupportingTabsInSidePanel });
+    },
+    [updateSettings],
+  );
+
   const handleUseLegacyTerminalRendererChange = useCallback(
     (useLegacyTerminalRenderer: boolean) => {
       void updateSettings({ useLegacyTerminalRenderer });
@@ -1420,6 +1447,7 @@ export default function SettingsScreen({ view, openAddHostIntent = null }: Setti
                 handleServiceUrlBehaviorChange={handleServiceUrlBehaviorChange}
                 handleLanguageChange={handleLanguageChange}
                 handleTerminalScrollbackLinesChange={handleTerminalScrollbackLinesChange}
+                handleSidePanelRoutingChange={handleSidePanelRoutingChange}
               />
               {isDesktopApp ? <BrowserDataSection /> : null}
             </>