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>

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>