Add profile timezone fallback for leave creation

[jernejk]

Summary

• Fixes Bug - Error 500 while user does not have any timezone set #10 by making leave creation resolve timezone in this order: explicit override, TimePro user profile timezone, then CLI/MCP host machine timezone.
• Adds tp leave create --timezone <TIMEZONE_ID> and MCP create_leave timeZoneId support for agent-controlled timezone overrides.
• Keeps the machine timezone fallback for profiles without timezone data, so the fallback is controlled by the runtime environment rather than automatic browser state.
• Adds an MCP create_leave path that shares the same parsing and timezone behavior as the CLI.

Sanity check

• Azure DevOps PR 3427 is completed/merged server-side, and staging no longer returns the previous null-timezone 500 in the duplicate-create smoke check.
• This PR keeps the tools side resilient rather than relying on the server alone.

Validation

• dotnet build --no-restore
• dotnet test --no-build
• dotnet run --no-build --project src/SSW.TimePro.Cli -- leave create --help
• git diff --check