feat: Hijri calendar, virtual midnight crossover, overlap rejection, WAL mode

- Hijri (قمری) calendar added as 3rd calendar option with tabular conversion
- Calendar type select menu (like accent) with gregorian/jalali/hijri
- Calendar type applied to exports and reports
- Midnight crossover: virtual splits at computation time (no synthetic DB events)
  - Handles both UTC and local timezone boundaries
  - Timezone changes handled naturally (recomputed on-the-fly)
- Rate limiting per user ID instead of chat ID
- WAL mode enabled for SQLite
- Smart reply keyboard updates after every state change
- Overlapping time edits rejected (must preserve in/out alternation)
- Single event deletion warns if timeline needs repair
- Day off no longer prevents clock in/out
- Accent colors shown with descriptive names in select menu
- Delete button label changed to DEL for visual distinction

db/schema.sql

@@ -17,6 +17,8 @@ CREATE TABLE IF NOT EXISTS users (
     last_report_date TEXT DEFAULT NULL,
     export_accent TEXT NOT NULL DEFAULT 'ocean'
         CHECK (export_accent IN ('ocean', 'beach', 'rose', 'catppuccin')),
+    calendar TEXT NOT NULL DEFAULT 'gregorian'
+        CHECK (calendar IN ('gregorian', 'jalali', 'hijri')),
     created_at INTEGER NOT NULL DEFAULT (unixepoch()),
     updated_at INTEGER NOT NULL DEFAULT (unixepoch())
 );