feat: make month week start configurable

Add a Monday-default --week-start option for every weekday and carry it through domain layout, full and compact rendering, event query windows, print mode, and the interactive TUI.

Preserve byte-compatible Monday output and cover rotated headers, spill dates, invalid input, sanitizer behavior, and live terminal restoration.

docs/ROADMAP.md

@@ -12,6 +12,7 @@
 - Occurrence-aware appointment search with exact result-to-grid navigation
 - Session calendar toggles shared by month rendering, focus, and search
 - Read-only 42-day agenda with bounded recurrence and exact return navigation
+- Configurable week start with Monday-default CLI and consistent grid queries
 - Explicit atomic import/export with validation, collision refusal, and backups
 - Meson build, Nix development shell, desktop entry, and Hyprland launcher
 - Seeded sample data when explicitly requested, never silent data mutation
@@ -19,7 +20,6 @@
 ## 0.1 — complete local calendar
 
 - Advanced recurrence editing, RDATE/overrides, and embedded VTIMEZONE support
-- Configurable week start
 - Screen-reader-friendly linear view and full Unicode display-width handling
 
 ## 0.2 — durable cache and provider boundary