fix: allow dates before unix epoch

[kaluli123123]

Summary

• allow the date picker to select dates before the Unix epoch, down to 1900
• represent validation bounds in local time to match parsed calendar dates
• cover the exact lower boundary, 1900-01-01, in a positive UTC offset timezone

Closes #8681

Validation

• flutter test --no-pub test/widget_test/date_picker_test.dart — 13 tests passed
• flutter analyze --no-pub . — no issues found
• Dart formatting and git diff --check pass

Validated with Flutter 3.27.4 on macOS arm64 (UTC+08:00).

[sourcery-ai]

Reviewer's Guide

Expands date picker support to historical dates by changing the lower boundary from 1970 to 1900 and adds widget coverage proving that a 1958 date can be entered and stored consistently with the picker boundary.

Flow diagram for historical date selection

flowchart LR
    Input["User enters or selects a date"] --> Boundary["Date picker boundary: 1900-2100"]
    Boundary --> Valid["1958 date accepted"]
    Valid --> Stored["Date stored consistently"]

Loading

File-Level Changes

Change	Details	Files
Moves the date picker’s earliest selectable date from the Unix epoch back to 1900.	Updates the calendar lower-bound constant to DateTime.utc(1900). Retains the existing 2100 upper bound while documenting the historical-date rationale.	frontend/appflowy_flutter/lib/workspace/presentation/widgets/date_picker/widgets/date_picker.dart
Adds regression coverage for selecting and validating a pre-epoch date through the date picker.	Verifies the calendar exposes 1900 as its first day. Enters and submits January 25, 1958 through the date field. Confirms the resulting application state contains the expected date.	frontend/appflowy_flutter/test/widget_test/date_picker_test.dart

Assessment against linked issues

Issue	Objective	Addressed	Explanation
#8681	Allow the date picker to select dates before the Unix epoch, including dates such as 25 January 1958.	✅
#8681	Keep text-based date entry validation consistent with the date picker's supported date range.	✅
#8681	Add regression coverage confirming that a pre-1970 date can be entered and stored correctly.	✅

Possibly linked issues

• [Bug] Date field can't go further back past 1970 (needed for inclusion of dates of birth) #8681: The PR directly fixes the reported pre-1970 date limitation and adds a regression test for a 1958 date.

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[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="frontend/appflowy_flutter/lib/workspace/presentation/widgets/date_picker/widgets/date_picker.dart" line_range="7-10" />
<code_context>
 import 'package:table_calendar/table_calendar.dart';
 import 'package:universal_platform/universal_platform.dart';

-final kFirstDay = DateTime.utc(1970);
+// Dates are also used for values such as birthdays, which can predate Unix
+// epoch. Keep the lower bound broad enough for historical dates while avoiding
+// the impractically large range before the Gregorian calendar.
+final kFirstDay = DateTime.utc(1900);
 final kLastDay = DateTime.utc(2100);

</code_context>
<issue_to_address>
**issue (bug_risk):** The UTC lower bound is compared with local `DateTime` values, so users in time zones east of UTC cannot enter or select 1 January 1900: local midnight is still 31 December 1899 UTC and is treated as before `kFirstDay`. The same date-only boundary mismatch affects the text-field validation that reuses this constant.

**Triggers:** When the user selects or enters the first day of the newly supported range in a positive-UTC-offset timezone.

**Suggested fix:** Compare date components rather than instants, or define the boundary in the same local date representation used by parsed and calendar dates.
</issue_to_address>

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