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/commit-diff-panel.spec.ts

@@ -51,6 +51,19 @@ test("commit history shows dates and shares diff layout preferences", async ({
   const panel = page.getByTestId("commit-diff-panel").filter({ visible: true });
   await expect(panel.getByTestId("commit-diff-toolbar")).toBeVisible({ timeout: 30_000 });
   const layoutToggle = panel.getByTestId("commit-diff-toggle-layout");
+  const [commitToolbarBox, layoutToggleBox, layoutToggleGlyphBox] = await Promise.all([
+    panel.getByTestId("commit-diff-header").boundingBox(),
+    layoutToggle.boundingBox(),
+    layoutToggle.locator("svg").boundingBox(),
+  ]);
+  if (!commitToolbarBox || !layoutToggleBox || !layoutToggleGlyphBox) {
+    throw new Error("Commit-diff toolbar geometry could not be measured");
+  }
+  expect(commitToolbarBox.height).toBe(36);
+  expect(layoutToggleBox.width).toBe(24);
+  expect(layoutToggleBox.height).toBe(24);
+  expect(layoutToggleGlyphBox.width).toBe(14);
+  expect(layoutToggleGlyphBox.height).toBe(14);
   await expect(layoutToggle).toHaveAccessibleName("Switch to side-by-side diff");
   await expect(panel.getByTestId("git-diff-canvas")).toBeVisible({ timeout: 30_000 });
   await expectCommitDiffHeaderGeometry(panel);