docs(18): create phase plan (4 plans, 3 waves)

.planning/ROADMAP.md

@@ -633,11 +633,23 @@ Plans:
 
 ### Phase 18: Auto timezone detection and ability to change timezone
 
-**Goal:** [To be planned]
-**Requirements**: TBD
-**Depends on:** Phase 17
-**Plans:** 0 plans
+**Goal:** Make the household timezone an explicit, stored, user-changeable setting — auto-detected from the browser at first run, changeable from the role-gated /admin Settings — and route the server-side all-day "9 AM local" reminder computation through it (replacing the implicit `process.env.TZ` fallback), without touching the already-correct browser-local display/timed-write path.
+**Requirements**: TBD (decision contract D-01..D-07 from 18-CONTEXT.md)
+**Depends on:** Phase 10 (admin role + `/admin` Settings + `app_config`); Phase 11 (all-day reminder computation this rewires). Independent of Phase 17. Phase 12 (setup wizard) not required — seeding is self-contained.
+**Plans:** 4 plans (3 waves)
 
 Plans:
+**Wave 1**
 
-- [ ] TBD (run /gsd-plan-phase 18 to break down)
+- [ ] 18-01-PLAN.md — TDD: getHouseholdTimezone(db) accessor + isValidIanaTimezone (D-05/D-06)
+
+**Wave 2** *(blocked on Wave 1 completion)*
+
+- [ ] 18-02-PLAN.md — TDD: admin GET/PUT/seed timezone endpoints on adminRouter, requireAdmin + IANA validation + no-overwrite seed (D-01/D-02/D-03/D-04)
+- [ ] 18-03-PLAN.md — TDD: route all-day reminder TZ at reminderScheduler:247 + outboxWorker:501,607 through the accessor (D-05/D-06/D-07)
+
+**Wave 3** *(blocked on Wave 2 completion)*
+
+- [ ] 18-04-PLAN.md — PWA Timezone section in /admin Settings (searchable IANA picker + detected-zone seed) + client fns (D-02/D-04)
+
+**UI hint**: yes