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/e2e/browser/explorer-surface-upgrade.spec.ts

@@ -24,11 +24,9 @@ function dockedSidebar(page: Page): Locator {
   return visible(page, "explorer-header");
 }
 
-/** The explorer pane is the tab row carrying the Changes tab. */
-function explorerTabRow(page: Page): Locator {
-  return visible(page, "workspace-tabs-row").filter({
-    has: page.getByTestId("workspace-tab-working_diff"),
-  });
+/** The desktop companion surface is a hideable side panel. */
+function sidePanel(page: Page): Locator {
+  return visible(page, "workspace-side-panel");
 }
 
 /** Seed exactly what a pre-pane build wrote after the user opened the sidebar once. */
@@ -72,11 +70,12 @@ test.describe("explorer surface after upgrading from the docked sidebar", () =>
 
         const toggle = visible(page, "workspace-explorer-toggle").first();
         await toggle.click();
-        await expect(explorerTabRow(page)).toHaveCount(1, { timeout: 15_000 });
+        await expect(sidePanel(page)).toHaveCount(1, { timeout: 15_000 });
+        await expect(sidePanel(page).getByTestId("workspace-pane-empty-state")).toBeVisible();
         await expect(dockedSidebar(page)).toHaveCount(0);
 
         await toggle.click();
-        await expect(explorerTabRow(page)).toHaveCount(0, { timeout: 15_000 });
+        await expect(sidePanel(page)).toHaveCount(0, { timeout: 15_000 });
         await expect(dockedSidebar(page)).toHaveCount(0);
       });
 
@@ -108,20 +107,22 @@ test.describe("explorer surface after upgrading from the docked sidebar", () =>
 
       await test.step("open the explorer so both sides are showing", async () => {
         await toggle.click();
-        await expect(explorerTabRow(page)).toHaveCount(1, { timeout: 15_000 });
+        await expect(sidePanel(page)).toHaveCount(1, { timeout: 15_000 });
+        await expect(sidePanel(page).getByTestId("workspace-pane-empty-state")).toBeVisible();
         await expect(agentList).toHaveCount(1);
       });
 
       await test.step("the shortcut collapses both", async () => {
         await page.keyboard.press(`${modifier}+.`);
-        await expect(explorerTabRow(page)).toHaveCount(0, { timeout: 15_000 });
+        await expect(sidePanel(page)).toHaveCount(0, { timeout: 15_000 });
         await expect(agentList).toHaveCount(0);
       });
 
       await test.step("the shortcut restores both", async () => {
         await page.keyboard.press(`${modifier}+.`);
         await expect(agentList).toHaveCount(1, { timeout: 15_000 });
-        await expect(explorerTabRow(page)).toHaveCount(1, { timeout: 15_000 });
+        await expect(sidePanel(page)).toHaveCount(1, { timeout: 15_000 });
+        await expect(sidePanel(page).getByTestId("workspace-pane-empty-state")).toBeVisible();
       });
     } finally {
       await workspace.cleanup();