Fix malformed race clock format and refresh docs screenshots

formatRaceClock rendered a doubled-up, garbled time (e.g.
"02:48.20:207") everywhere a race clock is shown - Timing, Overlay,
Results archive, Judging, Training stint logs - because it
concatenated both a centiseconds and a milliseconds computation into
the same string. Only the milliseconds part was ever meant to be
there. Found while building realistic demo data for new screenshots,
since every prior test/demo session was empty and never exercised
this code path with a real result.

Also regenerates all screenshots in docs/screenshots against a
realistic seeded race (drivers, cars, qualifying heats, finals,
practice history, a real judging correction) instead of empty demo
data, adds two new ones (Managed AMMC config, the in-app Guide page)
aimed at helping a new user understand initial setup, removes 6
screenshots orphaned by an earlier README revision, and updates both
README.md and README.sv.md's screenshot tables accordingly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.12.1",
+  "version": "1.12.2",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {