feat(PBI-76): migrate cookie-based prefs to user-settings (Phase 2) (#189)

* 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>

* feat(PBI-76): migrate SplitPane positions to user-settings store

Outside of a drag the store is the source of truth (cross-tab
updates flow in for free). During a drag we keep splits in local
state so mousemove does not round-trip through the store. On
mouseup we persist the final splits via setPref. Removes
document.cookie reads/writes — cookieKey is reused as the
store-key for backwards compat.

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

* feat(PBI-76): resolveActiveSprint reads from User.settings

lib/active-sprint:
- New helpers: getActiveSprintIdFromSettings, setActiveSprintInSettings,
  clearActiveSprintInSettings — all read/write user.settings.layout.activeSprints.
- resolveActiveSprint(productId, userId) — userId now required, falls back
  to first OPEN, then most recent CLOSED sprint.
- Cookie helpers (getActiveSprintIdFromCookie/setActiveSprintCookie/
  clearActiveSprintCookie) removed.

Callers updated to pass session.userId. The cookie-based fallback path
is gone — `actions/active-sprint.ts` and `actions/sprints.ts` will be
updated in the next commit (T-917).

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

* feat(PBI-76): rewrite setActiveSprint callers to use settings

setActiveSprintAction, syncActiveSprintCookieAction, and the two
sprint-creation paths in actions/sprints.ts now write through
setActiveSprintInSettings (which also emits pg_notify for cross-tab
sync) instead of dropping a cookie. The action names keep the
'cookie' suffix in the user-visible API for now — clean rename can
come later.

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

* 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>

* test(PBI-76): align tests with new SplitPane and active-sprint flow

- split-pane.test.tsx: seed positions via Zustand store instead of
  document.cookie; mock @/actions/user-settings so the prisma client
  is not transitively initialised in jsdom.
- backlog-split-pane.test.tsx: same action mock.
- sprint-dates.test.ts: add user.findUnique/update + $executeRaw
  mocks because createSprintAction now writes to user-settings.

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

---------

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

components/split-pane/split-pane.tsx

@@ -3,34 +3,15 @@
 import { Fragment, useRef, useState, useEffect, useCallback } from 'react'
 import { cn } from '@/lib/utils'
 import { debugProps } from '@/lib/debug'
+import { useUserSettingsStore } from '@/stores/user-settings/store'
 
-const COOKIE_PREFIX = 'sp:'
-const COOKIE_MAX_AGE = 60 * 60 * 24 * 365
-
-function readSplits(cookieKey: string, n: number): number[] | null {
-  if (typeof document === 'undefined') return null
-  const match = document.cookie.match(
-    new RegExp(`(?:^|; )${COOKIE_PREFIX}${cookieKey}=([^;]+)`)
+function isValidPositions(value: unknown, n: number): value is number[] {
+  return (
+    Array.isArray(value) &&
+    value.length === n &&
+    value.every((v) => typeof v === 'number') &&
+    Math.abs((value as number[]).reduce((a, b) => a + b, 0) - 100) <= 1
   )
-  if (!match) return null
-  try {
-    const parsed: unknown = JSON.parse(decodeURIComponent(match[1]))
-    if (
-      !Array.isArray(parsed) ||
-      parsed.length !== n ||
-      parsed.some((v) => typeof v !== 'number') ||
-      Math.abs((parsed as number[]).reduce((a, b) => a + b, 0) - 100) > 1
-    ) return null
-    return parsed as number[]
-  } catch {
-    return null
-  }
-}
-
-function writeSplits(cookieKey: string, splits: number[]) {
-  document.cookie = `${COOKIE_PREFIX}${cookieKey}=${encodeURIComponent(
-    JSON.stringify(splits)
-  )}; max-age=${COOKIE_MAX_AGE}; path=/; samesite=lax`
 }
 
 export interface SplitPaneProps {
@@ -59,9 +40,16 @@ export function SplitPane({
   const containerRef = useRef<HTMLDivElement>(null)
   const splitsRef = useRef<number[]>(defaultSplit)
 
-  const [splits, setSplits] = useState<number[]>(() => {
-    return readSplits(cookieKey, n) ?? defaultSplit
-  })
+  const persisted = useUserSettingsStore(
+    (s) => s.entities.settings.layout?.splitPanePositions?.[cookieKey],
+  )
+  const setPref = useUserSettingsStore((s) => s.setPref)
+
+  // While dragging we keep splits in local state to avoid round-tripping every
+  // mousemove through the store. Outside of a drag, the store is the source of
+  // truth so cross-tab updates flow in automatically.
+  const [dragSplits, setDragSplits] = useState<number[] | null>(null)
+  const splits = dragSplits ?? (isValidPositions(persisted, n) ? persisted : defaultSplit)
   const [dragging, setDragging] = useState<number | null>(null) // divider index (0..n-2)
   const [isMobile, setIsMobile] = useState(false)
   const [internalTab, setInternalTab] = useState(0)
@@ -96,20 +84,20 @@ export function SplitPane({
     const newLeft = Math.min(Math.max(cursorPct - leftEdge, minPct), combinedWidth - minPct)
     const newRight = combinedWidth - newLeft
 
-    setSplits((prev) => {
-      const next = [...prev]
-      next[dragging] = newLeft
-      next[dragging + 1] = newRight
-      return next
-    })
+    const base = splitsRef.current
+    const next = [...base]
+    next[dragging] = newLeft
+    next[dragging + 1] = newRight
+    setDragSplits(next)
   }, [dragging, minSize])
 
   const onMouseUp = useCallback(() => {
     if (dragging !== null) {
-      writeSplits(cookieKey, splitsRef.current)
+      void setPref(['layout', 'splitPanePositions', cookieKey], splitsRef.current)
+      setDragSplits(null)
       setDragging(null)
     }
-  }, [dragging, cookieKey])
+  }, [dragging, cookieKey, setPref])
 
   useEffect(() => {
     if (dragging !== null) {