Fix pcapng timestamp resolution + imported-session duration

- pcap.py: _parse_tsresol now pads options to a 32-bit boundary
  ((-length) % 4) and uses 2** for the MSB-set (power-of-2) case, so
  nanosecond if_tsresol is read correctly (was defaulting to µs and
  scaling timestamps 1000x)
- database/server: set imported session started/ended from the first/last
  packet timestamps (falling back to wall-clock for synthetic SPB)
- frontend: formatDuration renders hours
- tests: tsresol (ns after padded option, default µs, ns round-trip) and
  session-window/fallback (119 total)

simtrace2_pysniff/server/database.py

@@ -37,6 +37,11 @@ def _iso_now():
         f'{time.time() % 1 * 1000:03.0f}'
 
 
+def _iso_from_ts(ts):
+    return time.strftime('%Y-%m-%dT%H:%M:%S.', time.localtime(ts)) + \
+        f'{ts % 1 * 1000:03.0f}'
+
+
 class Database:
     def __init__(self, db_path=None):
         if db_path is None:
@@ -64,6 +69,12 @@ class Database:
             (_iso_now(), session_id))
         self._conn.commit()
 
+    def set_session_times_from_ts(self, session_id, first_ts, last_ts):
+        self._conn.execute(
+            'UPDATE sessions SET started=?, ended=? WHERE id=?',
+            (_iso_from_ts(first_ts), _iso_from_ts(last_ts), session_id))
+        self._conn.commit()
+
     def rename_session(self, session_id, name):
         self._conn.execute(
             'UPDATE sessions SET name=? WHERE id=?',