Navigation - add "Training" section

[gregoryfoster]

In the side bar navigation, add a "Training" section below the "Providers" section. Position the "Batches" and "Candidates" navigation links inside that section, in that order.

[gregoryfoster]

Moved Batches and Candidates into a new Training section in the sidebar nav, positioned after Providers.

[gregoryfoster]

Implemented in 7d56fef.

• Removed Batches and Candidates from the top-level nav list
• Added a new training_items group with label Training, ordered Batches → Candidates
• Appended after the Providers section in all_nav_items — existing group-header rendering handles the new section in both desktop sidebar and mobile nav