split of big plan

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+# Phase 2: Calendar State (`src/calendar/calendar-state.ts`)
+
+**Status:** Not started
+**Depends on:** Phase 1 (daily-notes.ts — for `indexDailyNotes`, `getDateFromDailyNote`)
+**Output file:** `src/calendar/calendar-state.ts`
+
+---
+
+## Overview
+
+Simple state container with change notifications — replaces Svelte stores. Holds the displayed month, today's date, active file tracking, and the note index. Notifies subscribers on any state change.
+
+---
+
+## Design
+
+```
+class CalendarState:
+  - displayedMonth: Moment (current month being viewed)
+  - today: Moment (refreshed by heartbeat)
+  - activeFileDate: string | null (date UID of active file, if daily note)
+  - noteIndex: Map<string, TFile> (date string → file)
+  - listeners: Set<() => void>
+
+  Methods:
+  - subscribe(cb): () => void (unsubscribe function)
+  - setDisplayedMonth(m: Moment): void
+  - setActiveFile(file: TFile | null): void
+  - reindex(app: App, rootFolder: string): void
+  - tick(): void (refresh today)
+  - notify(): void (call all listeners)
+```
+
+---
+
+## Notes
+
+- `reindex()` delegates to `indexDailyNotes()` from Phase 1.
+- `setActiveFile()` uses `getDateFromDailyNote()` from Phase 1 to determine if the file is a daily note.
+- `tick()` updates `today` and calls `notify()` only if the date has changed (day rollover).
+- Subscribers are plain callbacks — no framework dependency.