Calendar: fix weekday-header/week-number alignment to firstWeekday + tests

Root cause: a data-integrity bug, not visual. The month grid is built from
.weekOfMonth (respects Calendar.current.firstWeekday), but the weekday header
was hardcoded Sunday-first ("S M T W T F S") and the week-number label was gated
on isSunday (weekday == 1). On any non-Sunday-first locale (or when the user sets
"Start Week On" = Monday) every column was mislabeled by one and the W-number
landed on the wrong column — e.g. June 1 2026 (a Monday) appearing under "S".

Fix:
- Extract pure, testable date math into CalendarGrid (monthGrid + weekdaySymbols).
- Header now derived from firstWeekday, so it always matches the grid.
- Week-number label gated on isWeekStart (weekday == firstWeekday), not Sunday.
- Document that week numbers are locale weekOfYear (consistent with the layout),
  not ISO.
- gridDays() and the year-view mini-month dedupe to CalendarGrid.

Verification:
- New KisaniCalTests target with 9 tests, all passing on simulator: exact
  Sunday-first June 2026 grid, header↔grid alignment for firstWeekday 1...7,
  cell date identity, month navigation (May/Jul/Feb 2026 + Feb 2028 leap +
  Dec 2026→Jan 2027), and event-day bucketing (all-day, late-night, midnight-
  crossing) across 4 timezones.
- Verified in the running app: header S M T W T F S, May 31 in the Sunday
  column, June 1 under Monday, June 24 (Wednesday) selected.

Adds a DEBUG-only KISANI_INITIAL_TAB launch env (compiled out of Release) used
to screenshot the calendar past the sign-in gate.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

KisaniCal/ContentView.swift

@@ -11,7 +11,12 @@ struct ContentView: View {
     @AppStorage("showWorkoutTab")  private var showWorkoutTab  = true
     @AppStorage("hasOnboarded")    private var hasOnboarded    = false
     @State private var showOnboarding    = false
-    @State private var selectedTab       = 0
+    @State private var selectedTab       = {
+        #if DEBUG
+        if let t = ProcessInfo.processInfo.environment["KISANI_INITIAL_TAB"], let i = Int(t) { return i }
+        #endif
+        return 0
+    }()
     @State private var showQuickAddTask  = false
     @StateObject private var tabState    = FloatingTabState()
     @ObservedObject private var shortcuts = ShortcutHandler.shared