Simplify skipped scenario format with placeholder step

Replace @skip tag + placeholder step with just placeholder step.
Skipped scenarios now only need:
  Scénario: Name
    * Scénario non implémenté

- Remove @skip tags from all 26 feature files
- Add step definition returning 'skipped' for placeholder
- Update GherkinHighlighter to hide placeholder and make
  skipped scenarios non-expandable (no chevron, no click)
- Update documentation with new format

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

features/step_definitions/navigation.steps.ts

@@ -2,6 +2,12 @@ import { Given, When, Then } from '@cucumber/cucumber';
 import { expect } from 'chai';
 import type { FestipodWorld } from '../support/world';
 
+// Placeholder step for scenarios that are not yet implemented
+// This step indicates the feature is planned but not built yet
+Given('Scénario non implémenté', async function (this: FestipodWorld) {
+  return 'skipped';
+});
+
 const screenNameMap: Record<string, string> = {
   'accueil': 'home',
   'liste des événements': 'events',