refactor(settings): scope the settings project selector to settings

Picking a project in Settings called setActiveProject, which relocates
the chat, the session list, the file tree and the Git surface. Reading
another project's MCP servers or agents moved the user's whole app.

It had to, because the configuration stores resolved the directory
themselves from the active project and held one flat list. Each of them
now takes an explicit directory — omitted still means the active project,
so every caller outside Settings is unchanged — and keys loaded data by
directory next to a flat mirror of the active project. Chat, autocompletes
and pickers keep reading that mirror; a load for another directory writes
only the map. A failed load restores that directory's previous list.

Settings resolves its own directory through useSettingsDirectory, backed
by a session-local settingsProjectPath that follows the active project
until the user picks something else.

packages/ui/src/stores/useUIStore.ts

@@ -688,6 +688,13 @@ interface UIStore {
   settingsPage: string;
   settingsHasOpenedOnce: boolean;
   settingsProjectsSelectedId: string | null;
+  /**
+   * Project the Settings pages are looking at. `null` follows the app's active
+   * project. Settings browses another project's configuration without moving
+   * the chat, the session list or the file tree, so this is its own state and
+   * not a second writer of the active project.
+   */
+  settingsProjectPath: string | null;
   settingsRemoteInstancesSelectedId: string | null;
   eventStreamStatus: EventStreamStatus;
   eventStreamHint: string | null;
@@ -883,6 +890,7 @@ interface UIStore {
   setSidebarSection: (section: SidebarSection) => void;
   setSettingsPage: (slug: string) => void;
   setSettingsProjectsSelectedId: (projectId: string | null) => void;
+  setSettingsProjectPath: (path: string | null) => void;
   setSettingsRemoteInstancesSelectedId: (instanceId: string | null) => void;
   setEventStreamStatus: (status: EventStreamStatus, hint?: string | null) => void;
   setShowReasoningTraces: (value: boolean) => void;
@@ -1055,6 +1063,7 @@ export const useUIStore = create<UIStore>()(
         settingsPage: 'home',
         settingsHasOpenedOnce: false,
         settingsProjectsSelectedId: null,
+        settingsProjectPath: null,
         settingsRemoteInstancesSelectedId: null,
         eventStreamStatus: 'idle',
         eventStreamHint: null,
@@ -1820,6 +1829,11 @@ export const useUIStore = create<UIStore>()(
           set({ settingsPage: slug });
         },
 
+        setSettingsProjectPath: (path) => {
+          const trimmed = path?.trim();
+          set({ settingsProjectPath: trimmed ? trimmed : null });
+        },
+
         setSettingsProjectsSelectedId: (projectId) => {
           set({ settingsProjectsSelectedId: projectId });
         },