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.

docs/development.md

@@ -242,13 +242,13 @@ focused invalidation capture; React Native's generated stacks make that mode hig
 Set `PASEO_PROFILE_TRACE_FOCUS=1` to include focus targets, durations, and JavaScript call stacks in
 the scenario report. This mode wraps `HTMLElement.focus`, so use it only for diagnosis.
 
-For the desktop explorer toggle, run the app against the root checkout's daemon and use:
+For the desktop Side panel toggle, run the app against the root checkout's daemon and use:
 
 ```bash
 npm run profile:explorer-toggle --workspace=@getpaseo/app
 ```
 
-The harness verifies port `6768`, opens the Paseo workspace, creates and warms the explorer pane,
+The harness verifies port `6768`, opens the Paseo workspace, creates and warms the Side panel pane,
 records an idle control, then measures settled and 50 ms burst Cmd+E toggles. It reports
 input-to-DOM and input-to-paint latency, React commits, mounts, unmounts, and DOM mutations. Set
 `PASEO_PROFILE_TRACE_PATH=/tmp/explorer-toggle.trace.json` or