Print the actual reason a --changed-before/--changed-within date fails to parse

[MsfPablo]

Closes #2053.

--changed-before / --changed-within currently swallow the parse error and print the same generic message for every bad input:

$ fd -u --changed-before=2025-11-31
[fd error]: '2025-11-31' is not a valid date or duration. See 'fd --help'.

As the issue points out, that reads like a formatting problem, when the real problem is that November has 30 days.

TimeFilter::from_str discarded the jiff errors because it returned Option<SystemTime>. This changes TimeFilter::before/after to return Result<TimeFilter, String> and passes the reason through to the user:

$ fd -u --changed-before=2025-11-31
[fd error]: '2025-11-31' is not a valid date or duration: parsed date is not valid: parameter 'day' for `2025-11` is invalid, must be in range `1..=30`. See 'fd --help'.

$ fd --changed-within=foo
[fd error]: 'foo' is not a valid date or duration: failed to parse year in date: expected four digit year (or leading sign for six digit year), but found end of input. See 'fd --help'.

$ fd --changed-within=@x
[fd error]: '@x' is not a valid unix timestamp: expected '@' followed by a number of seconds since the epoch. See 'fd --help'.

The @-timestamp branch was also moved up so that an input starting with @ reports a timestamp error rather than falling through to the date parser and complaining about years.

Accepted inputs are unchanged; only the error paths differ. Added a unit test asserting the new messages, and updated the existing tests plus the CHANGELOG.

Verification (clean, and also clean on a git stash baseline):

cargo test                                  # 158 + 108 passed, 0 failed
cargo fmt --check                           # no output
cargo clippy --all-targets -- -D warnings   # no warnings

AI disclosure

Per CONTRIBUTING.md item 4: I used an AI coding assistant (Claude Code) for this change. It was used to locate the code path, draft the Option -> Result refactor in src/filter/time.rs and the matching call sites in src/main.rs, and to draft the unit test. I reviewed the diff, ran the commands above myself, and checked the resulting messages against the cases in the issue. I understand the change and can answer questions or revise it. The PR text is my own; AI was not used to write this description beyond assembling the command output above.