fix(time): show dates for files modified before the UNIX epoch

[xfocus3]

Problem

Files whose modification (or access/change) time is before the UNIX epoch (1970-01-01) render their date as - in eza, even though ls shows the correct date (e.g. 1901-12-13 ...). See #1668.

Root cause

File::systemtime_to_naivedatetime in src/fs/file.rs did:

let duration = st.duration_since(SystemTime::UNIX_EPOCH).ok()?;

SystemTime::duration_since returns Err whenever the time is earlier than the epoch, and .ok()? silently converted that Err into None. Every pre-epoch timestamp therefore became "no date", which the table renderer prints as -.

Fix

Handle the Err branch instead of discarding it:

• On Ok(d): behave as before (positive seconds + nanos).
• On Err(e): recover the absolute offset from e.duration(), negate the seconds, and when there is a sub-second part floor towards negative infinity (secs -= 1; nanos = 1_000_000_000 - nanos) so the value matches how Unix timestamps count time before 1970.

The timestamp is then built with chrono::DateTime::from_timestamp(secs, nanos), which already accepts negative seconds, keeping the existing Option<NaiveDateTime> return type and .naive_local() conversion. The change is localized to this one function. ls is treated as the ground-truth behavior.

Tests

Added a #[cfg(test)] mod systemtime_to_naivedatetime_test covering:

• a normal post-epoch time (still works, year 2001),
• the epoch itself (timestamp 0, year 1970),
• a pre-epoch time (UNIX_EPOCH - 2_147_483_648s, year 1901, timestamp -2_147_483_648),
• a pre-epoch time with a sub-second part (verifies the floor-towards-negative-infinity handling: -10.25s -> timestamp -11, nanos 750_000_000).

cargo build, cargo test fs::file, cargo clippy -- -D warnings, and cargo fmt --check all pass locally on the pinned toolchain (1.90).

Closes #1668