Stability hardening: crash.log + disk-full preflight (2.5)

Two independent improvements from Phase 2.5.

## Panic hook → crash.log

GUI users launched from a .desktop file have no stderr — today panics
vanish without trace. Install a panic::set_hook early in main() that
appends a structured entry to <library_root>/yt-offline.crash.log for
every panic from every thread (UI, axum workers, downloader spawns,
tray bg thread, thumbnail decode workers).

Each entry: ISO-8601 timestamp, thread name, file:line, panic message,
and a backtrace when RUST_BACKTRACE is set. The default panic hook
still runs after ours, so dev workflows keep getting the stderr trace.

Bounded: when the log passes 256 KB the next write rotates it to .1
(one previous generation kept). No external date library — small
civil-time formula in-tree.

## Disk-full preflight

Before yt-dlp ever launches, statvfs the target filesystem. If free
space is under 500 MB, push a synthetic Failed job classified as
DiskFull instead of starting yt-dlp. That way:

- Users see the actual problem ("only 230 MB free on /mnt/…") in the
  Downloads modal with the same "free up space and retry" hint the
  error-classifier emits for mid-download ENOSPC.
- No half-written file lands on disk.
- No 30-second yt-dlp warmup wasted on a doomed run.

statvfs returning None (non-Unix, missing path) skips the check —
better to let yt-dlp run than to refuse on no data.

5 new tests: 3 in crash (civil-time, log shape, real-panic capture),
2 in disk_space (statvfs plumbing + missing-path None). 74 pass total.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.toml

@@ -25,6 +25,7 @@ rand = "0.8"
 # (zbus, no GTK/libdbus build dep), consistent with notify-rust above.
 rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "tokio"] }
 ksni = { version = "0.3.4", features = ["tokio"] }
+libc = "0.2.186"
 
 [profile.release]
 opt-level = 2