Add multi-day calendar selection with custom popover UI

[iamtoruk]

Summary

• Custom CalendarPopover replaces native DatePicker, themed to match the app
• Non-contiguous multi-day selection: pick any combination of days across months
• --days CLI flag with comma-separated dates, post-filters from bounding range scan
• Pending state committed only on Done; Clear + Done resets to active period
• Absorbs PR Add single-day review mode #380's day-review foundation (single-day mode, --from/--to, dashboard day nav)

What changed

CLI (src/):

• parseDaysFlag() in cli-date.ts: parses, validates, deduplicates comma-separated dates
• filterProjectsByDays() in parser.ts: post-filters turns to only selected days
• --days option on status command, wired into menubar-json with filtering on all paths (projects, providers, daily cache)
• --from/--to added to status for contiguous ranges
• Dashboard day-review mode from PR Add single-day review mode #380 (d key, arrow nav, DayBanner)

Swift menubar (mac/):

• PeriodSegmentedControl: calendar icon inline in period strip, custom CalendarPopover with month nav, multi-day toggle, Done/Clear buttons
• AppStore: selectedDays: Set<String>, switchTo(days:), PayloadCacheKey with days field
• DataClient: passes --days for multi-day, single-day fallback for days.count == 1

Verification

• 975 tests pass
• Integration verified: sum of individual --day calls matches --days output across cost, calls, activities (12 categories), models (9), and providers (12)
• Cross-month selection verified (Apr + May days)
• Council deep review with actual source: no real bugs found (2 found and fixed during review)

Test plan

• Open menubar, click calendar icon, select 2+ non-contiguous days, tap Done
• Verify cost/calls match sum of individual days
• Clear all days + Done returns to active period
• Select days across different months
• Single day selection works
• Dismiss popover without Done discards changes
• Period tabs still work normally after day selection