feat(chronik): extract feedFilters.ts with youParticipated in fuer-dich

Extract filterFeed(items, filter) from +page.svelte inline switch to a
pure function, widening the fuer-dich branch to include youParticipated.
Regenerate ActivityFeedItemDTO type to include the new field.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/generated/api.ts

@@ -2038,6 +2038,7 @@ export interface components {
             /** Format: date-time */
             happenedAt: string;
             youMentioned: boolean;
+            youParticipated: boolean;
             /** Format: int32 */
             count: number;
             /** Format: date-time */