Split the Guide page into tabs, matching the Settings/Event Manager pattern

The page had grown to ~20 stacked panels on one very long scroll - the same problem
already fixed twice this session for Settings and Event Manager. Grouped into 5 tabs
(Getting started, Create a race, Run a race, Training & results, Video guides) using
the same activeGuideTab-as-module-state approach so the selection survives an in-place
re-render (e.g. a language switch). The always-visible overview cards at the top are
unchanged.

Also investigated a "feels slow" concern raised alongside this: measured the page's
actual render cost in a real browser (median 0.6ms, on par with or faster than Settings/
Timing) and confirmed the periodic tickClock/updateSyncStatus intervals never re-render
an idle Guide page. No performance issue found - the perceived slowness was the page's
length, which this change addresses directly.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.20.2",
+  "version": "1.21.0",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {