refactor: normalize date handling to Tehran timezone

- Updated date handling in BlogSeoFields and ScheduleSection to use Tehran timezone utilities for consistency.
- Introduced `toTehranClockTime`, `tehranWallClockToUnix`, and `todayIso` functions for accurate date representation.
- Modified various components to utilize these new utilities, ensuring that date strings are correctly formatted and timestamps are accurately converted.
- Enhanced API documentation to clarify the handling of date fields, emphasizing the importance of server-local midnight.
- Added tests to verify that date overrides and holidays maintain the correct day without shifting due to timezone discrepancies.

docs/api/appointment-settings.md

@@ -312,6 +312,7 @@ Get all date overrides for a doctor.
         "uuid": "...",
         "doctor_uuid": "...",
         "date": 1718476800,
+        "date_string": "2024-06-15",
         "active": false,
         "reason": "تعطیل خاص",
         "custom_slots": [],
@@ -322,7 +323,7 @@ Get all date overrides for a doctor.
 }
 ```
 
-> `date` is a Unix timestamp. `active: false` = entire day blocked. `active: true` with `custom_slots` = custom session schedule.
+> `date` is a Unix timestamp at **server-local midnight** (`Asia/Tehran`); `date_string` is the same day as `Y-m-d` and is what clients must render — converting the timestamp in a browser with `toISOString()` (UTC) shifts it one day back. `active: false` = entire day blocked. `active: true` with `custom_slots` = custom session schedule.
 
 ---
 
@@ -387,6 +388,7 @@ Create a date override.
       "uuid": "override-uuid-...",
       "doctor_uuid": "...",
       "date": 1718476800,
+      "date_string": "2024-06-15",
       "active": false,
       "reason": "تعطیل رسمی",
       "custom_slots": [],
@@ -490,6 +492,8 @@ Get all holidays for a doctor.
         "doctor_uuid": "...",
         "start_date": 1719792000,
         "end_date": 1720656000,
+        "start_date_string": "2024-07-01",
+        "end_date_string": "2024-07-11",
         "reason": "تعطیلات تابستانی",
         "active": true,
         "created_at": 1717000000
@@ -499,7 +503,7 @@ Get all holidays for a doctor.
 }
 ```
 
-> `start_date` and `end_date` are Unix timestamps. Holidays take **highest priority** — they block the day even if a date override exists.
+> `start_date` / `end_date` are Unix timestamps at **server-local midnight** (`Asia/Tehran`); `start_date_string` / `end_date_string` carry the same days as `Y-m-d` and are what clients must render (UTC conversion in the browser shifts them one day back). Holidays take **highest priority** — they block the day even if a date override exists.
 
 ---
 
@@ -536,6 +540,8 @@ Create a holiday range.
       "doctor_uuid": "...",
       "start_date": 1719792000,
       "end_date": 1720656000,
+      "start_date_string": "2024-07-01",
+      "end_date_string": "2024-07-11",
       "reason": "تعطیلات تابستانی",
       "active": true,
       "created_at": 1717000000