fix(cli): prevent overflow panic in --since parsing

[atharva-patill]

Summary

While testing --since, I noticed that passing a very large day value (for example 500000000d) causes the CLI to panic because of unchecked date arithmetic.

This changes that behavior to return a normal user-facing error instead of panicking. I've also added a regression test to cover this case.

Before

$ ctx search foo --since 500000000d

thread 'main' panicked at crates/ctx-cli/src/main.rs:6417:19:
DateTime - TimeDelta overflowed

After

$ ctx search foo --since 500000000d

Error: invalid --since day window: 500000000d: value too large

Validation

• Added a regression test for oversized --since values
• Relevant unit tests pass

[luca-ctx]

Thank you for the bugfix!

[atharva-patill]

Thank you for the bugfix!

@luca-ctx You're welcome!