Move Academy from hidden Standards sub-tab to a dedicated
top-level tab '🎓 Academy' in the main navigation bar.
- Add <div class="tab" data-tab="training"> to nav
- Create standalone <div id="tab-training"> with full Academy HTML
- Wire initTraining() into goToTab() handler
- Remove std-subtab-training skeleton from Standards section
- Remove training button from Standards sub-tab bar
- Update switchStdSubtab() to only handle standards/formfaktoren
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