Add ISO week number tokens to the clock format

[danielchalmers]

Adds {week} (ISO 8601 week number) and {weekYear} (ISO week-numbering year) as custom clock format tokens, so formats like Week {week} or {weekYear}-W{week} work in the clock display.

Supersedes #130 (thanks @spacevortex2435 for the groundwork! token names and several test cases carry over):

• The ISO week math lives in DateTimeUtil as documented extension methods instead of private helpers inside the tokenizer (ISOWeek isn't available on .NET Framework, so it's hand-rolled with the same semantics).
• The tokens are discoverable: a Week number insert button in the inline format editor (Add inline format editor for clock and countdown #140) and examples in the format list.
• A countdown format using {week} shows the standard "Bad format" error instead of throwing, with a test covering it.

Details

• Week numbers respect the selected time zone since conversion happens before formatting (test included).
• Number output uses the active format provider like every other token.
• Around New Year, {weekYear} differs from {yyyy} per ISO 8601 — e.g. Dec 30, 2024 is 2025-W1.

Closes #130
Closes #107
Closes #78