feat(PBI-76): migration helper v2 — handle legacy cookies

Bumps marker version to 'v2'. buildMigrationPatch now also scans
document.cookie for `sp:*` (split-pane positions) and
`active_sprint_*` (active sprint per product) and lifts them into
layout.splitPanePositions / layout.activeSprints. clearLegacyStorage
replaces clearLegacyLocalStorage and clears both keys and cookies.
clearLegacyLocalStorage stays as a deprecated alias so the bridge
upgrade is a single rename.

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

lib/user-settings-migration.ts

@@ -8,14 +8,28 @@
 import type { UserSettings } from './user-settings'
 
 const MIGRATION_MARKER = 'scrum4me:settings_migrated'
-const CURRENT_VERSION = 'v1'
+const CURRENT_VERSION = 'v2'
 
 export interface MigrationResult {
   patch: Partial<UserSettings>
   legacyKeys: string[]
+  legacyCookies: string[]
   hasData: boolean
 }
 
+function readCookies(): Record<string, string> {
+  if (typeof document === 'undefined') return {}
+  const out: Record<string, string> = {}
+  for (const part of document.cookie.split(';')) {
+    const eq = part.indexOf('=')
+    if (eq < 0) continue
+    const key = part.slice(0, eq).trim()
+    const val = part.slice(eq + 1).trim()
+    if (key) out[key] = val
+  }
+  return out
+}
+
 function readJsonArray(key: string): string[] | null {
   const raw = localStorage.getItem(key)
   if (!raw) return null
@@ -54,13 +68,20 @@ function setIfNotNull<T>(target: Record<string, unknown>, key: string, value: T
 }
 
 export function buildMigrationPatch(): MigrationResult {
-  const empty: MigrationResult = { patch: {}, legacyKeys: [], hasData: false }
+  const empty: MigrationResult = {
+    patch: {},
+    legacyKeys: [],
+    legacyCookies: [],
+    hasData: false,
+  }
   if (typeof window === 'undefined') return empty
   if (localStorage.getItem(MIGRATION_MARKER) === CURRENT_VERSION) return empty
 
   const patch: Partial<UserSettings> = {}
   const views: NonNullable<UserSettings['views']> = {}
+  const layout: NonNullable<UserSettings['layout']> = {}
   const legacyKeys: string[] = []
+  const legacyCookies: string[] = []
   let hasData = false
 
   // sprint_pb_*
@@ -206,10 +227,48 @@ export function buildMigrationPatch(): MigrationResult {
     hasData = true
   }
 
-  return { patch, legacyKeys, hasData }
+  // layout from cookies (Phase 2)
+  const cookies = readCookies()
+  const splitPanePositions: Record<string, number[]> = {}
+  const activeSprints: Record<string, string> = {}
+  for (const [name, rawValue] of Object.entries(cookies)) {
+    if (name.startsWith('sp:')) {
+      const key = name.slice(3)
+      try {
+        const arr = JSON.parse(decodeURIComponent(rawValue))
+        if (
+          Array.isArray(arr) &&
+          arr.every((n) => typeof n === 'number') &&
+          Math.abs(arr.reduce((a, b) => a + b, 0) - 100) <= 1
+        ) {
+          splitPanePositions[key] = arr as number[]
+          legacyCookies.push(name)
+        }
+      } catch {
+        // ignore malformed cookie
+      }
+    } else if (name.startsWith('active_sprint_') && rawValue) {
+      const productId = name.slice('active_sprint_'.length)
+      activeSprints[productId] = decodeURIComponent(rawValue)
+      legacyCookies.push(name)
+    }
+  }
+  if (Object.keys(splitPanePositions).length > 0) {
+    layout.splitPanePositions = splitPanePositions
+    hasData = true
+  }
+  if (Object.keys(activeSprints).length > 0) {
+    layout.activeSprints = activeSprints
+    hasData = true
+  }
+  if (Object.keys(layout).length > 0) {
+    patch.layout = layout
+  }
+
+  return { patch, legacyKeys, legacyCookies, hasData }
 }
 
-export function clearLegacyLocalStorage(keys: string[]): void {
+export function clearLegacyStorage(keys: string[], cookies: string[] = []): void {
   if (typeof window === 'undefined') return
   for (const k of keys) {
     try {
@@ -218,9 +277,20 @@ export function clearLegacyLocalStorage(keys: string[]): void {
       // storage quota exceeded or disabled — ignore
     }
   }
+  for (const c of cookies) {
+    try {
+      document.cookie = `${c}=; max-age=0; path=/; samesite=lax`
+    } catch {
+      // ignore
+    }
+  }
   try {
     localStorage.setItem(MIGRATION_MARKER, CURRENT_VERSION)
   } catch {
     // ignore
   }
 }
+
+/** @deprecated use clearLegacyStorage */
+export const clearLegacyLocalStorage = (keys: string[]) =>
+  clearLegacyStorage(keys, [])