Graceful clipboard handling and timezone-correct dates

- Fix: invalid YAML header in clipboard/stdin treated as body — when the
clipboard or stdin stream contains syntactically invalid YAML between ---
delimiters, Tp-Note no longer aborts with an error. Instead a warning is
logged and the entire input is treated as plain body text, so note creation
continues normally.
- Fix: note filenames use local date instead of UTC — the date filter
previously computed dates from the Unix epoch in UTC, causing wrong dates in
filenames for users in timezones east of UTC (e.g. a note created at 02:00
EEST was dated the previous day). Dates now reflect the local timezone.
This regression was introduced with v1.26.2.