[Fix] Use proper rounding for symbol frequency statistics

[Fix] Use proper rounding for symbol frequency statistics

- Replace incorrect floor() with round() in rounding functions to avoid
  losing small values
- Increase counters API frequency precision from 3 to 6 decimal places
  (need 5 to avoid rspamc displaying values as multiples of 0.06, need 6
  for /counters endpoint itself - no additional overhead as JSON stores
  double anyway)
- Add frequency_stddev field to counters API output (fixes zero stdev in
  `rspamc counters` output)
- Clarify `rspamc counters` table header with "avg (stddev)" subheading
- Fix WebUI to preserve frequency precision before scaling

Example for symbol with frequency 0.004772 hits/sec:
- Before: /symbols returns 0.004772, /counters returns 0.004000,
  `rspamc counters` shows 0.240
- After:  /symbols returns 0.004772, /counters returns 0.004772,
  `rspamc counters` shows 0.286