chore: complete v1.1 milestone

Archive v1.1 Calendar & Polish milestone artifacts (roadmap,
requirements, phase directories) to milestones/. Evolve PROJECT.md
with validated requirements and new key decisions. Update
RETROSPECTIVE.md with v1.1 section and cross-milestone trends.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/MILESTONES.md

@@ -1,5 +1,22 @@
 # Milestones
 
+## v1.1 Calendar & Polish (Shipped: 2026-03-16)
+
+**Phases completed:** 3 phases, 5 plans, 11 tasks
+**Codebase:** 13,031 LOC Dart (9,051 lib + 3,980 test), 108 tests, 41 commits
+**Timeline:** 2 days (2026-03-15 to 2026-03-16)
+
+**Key accomplishments:**
+1. Horizontal 181-day calendar strip with German day cards, month boundaries, and floating Today button — replaces the stacked daily-plan HomeScreen
+2. Date-parameterized CalendarDao with reactive Drift streams for day tasks and overdue tasks
+3. Task completion history bottom sheet with per-task reverse-chronological log
+4. Alphabetical, interval, and effort sort options persisted via SharedPreferences
+5. SortDropdown widget integrated in both HomeScreen and TaskListScreen AppBars
+
+**Archive:** See `milestones/v1.1-ROADMAP.md` and `milestones/v1.1-REQUIREMENTS.md`
+
+---
+
 ## v1.0 MVP (Shipped: 2026-03-16)
 
 **Phases completed:** 4 phases, 13 plans