feat(PBI-76): extend UserSettings schema with layout

Adds layout.splitPanePositions and layout.activeSprints. These will
hold values currently kept in client-side and server-side cookies
(Phase 2). Two new tests cover the shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

__tests__/lib/user-settings.test.ts

@@ -109,7 +109,17 @@ describe('UserSettingsSchema', () => {
         jobsColumns: { 'queue:active': { kinds: ['TASK_IMPLEMENTATION'], statuses: [] } },
       },
       devTools: { debugMode: true },
+      layout: {
+        splitPanePositions: { 'backlog-pid': [25, 35, 40] },
+        activeSprints: { 'product-1': 'sprint-1' },
+      },
     })
     expect(result.success).toBe(true)
   })
+
+  it('accepts layout-only settings', () => {
+    expect(UserSettingsSchema.safeParse({
+      layout: { splitPanePositions: { x: [50, 50] }, activeSprints: { p: 's' } },
+    }).success).toBe(true)
+  })
 })

lib/user-settings.ts

@@ -43,9 +43,15 @@ const DevToolsPrefs = z.object({
   debugMode: z.boolean().optional(),
 }).strict()
 
+const LayoutPrefs = z.object({
+  splitPanePositions: z.record(z.string(), z.array(z.number())).optional(),
+  activeSprints: z.record(z.string(), z.string()).optional(),
+}).strict()
+
 export const UserSettingsSchema = z.object({
   views: ViewsPrefs.optional(),
   devTools: DevToolsPrefs.optional(),
+  layout: LayoutPrefs.optional(),
 }).strict()
 
 export type UserSettings = z.infer<typeof UserSettingsSchema>