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/i18n/resources/en.ts

@@ -551,7 +551,6 @@ export const en = {
       loading: "Loading...",
       modified: "Unsaved changes",
       loadingAgentTitle: "Loading agent title",
-      emptyPane: "No tabs in this pane.",
       fallback: {
         newAgent: "New agent",
         setup: "Setup",
@@ -593,6 +592,7 @@ export const en = {
         newBrowser: "New browser",
         maximizePane: "Maximize pane",
         restorePane: "Restore pane",
+        closePane: "Close pane",
         exitFocusMode: "Exit focus mode",
         splitRight: "Split pane right",
         splitDown: "Split pane down",
@@ -602,10 +602,10 @@ export const en = {
         terminalProfilesMenu: "Terminal profiles",
         editTerminalProfiles: "Edit profiles…",
       },
-      explorer: {
-        open: "Open explorer",
-        close: "Close explorer",
-        toggle: "Toggle explorer",
+      sidePanel: {
+        open: "Open side panel",
+        close: "Close side panel",
+        toggle: "Toggle side panel",
         changes: "Changes",
         files: "Files",
       },
@@ -706,6 +706,10 @@ export const en = {
         stopFailed: "Failed to stop {{scriptName}}",
       },
     },
+    tree: {
+      showFolderTree: "Show folder tree",
+      hideFolderTree: "Hide folder tree",
+    },
     git: {
       actions: {
         moreOptions: "More options",
@@ -864,8 +868,6 @@ export const en = {
         split: "Side-by-side diff",
         switchToUnified: "Switch to unified diff",
         switchToSplit: "Switch to side-by-side diff",
-        showTreeView: "Show folder tree",
-        hideTreeView: "Hide folder tree",
         options: "Diff options",
         hideWhitespace: "Hide whitespace",
         showWhitespace: "Show whitespace",
@@ -1981,6 +1983,11 @@ export const en = {
         description: "Lines kept in the built-in terminal buffer",
         accessibilityLabel: "Terminal scrollback lines",
       },
+      sidePanelRouting: {
+        label: "Open supporting tabs in the Side panel",
+        description:
+          "File links, pull requests, and setup progress open beside your work instead of in the focused pane",
+      },
       autoExpandReasoning: {
         label: "Always expand reasoning",
         description: "Show agent thinking and chain-of-thought blocks fully expanded by default",
@@ -2164,11 +2171,11 @@ export const en = {
         toggleCommandCenter: "Toggle command center",
         showKeyboardShortcuts: "Show keyboard shortcuts",
         toggleLeftSidebar: "Toggle left sidebar",
-        toggleRightSidebar: "Toggle right sidebar",
+        toggleRightSidebar: "Toggle side panel",
         toggleBothSidebars: "Toggle both sidebars",
         toggleSettings: "Toggle settings",
         toggleFocusMode: "Toggle focus mode",
-        toggleExplorerPaneMaximization: "Toggle Explorer pane maximization",
+        toggleExplorerPaneMaximization: "Toggle Side panel maximization",
         cycleTheme: "Cycle theme",
         focusMessageInput: "Focus message input",
         cycleAgentMode: "Cycle agent mode",