fix(voice,knowledge): mic recording + pagination + STT-only mode

Voice: recreate AudioRecorder each session to avoid stale native
state, improve permission handling (re-check after settings), show
feedback after 3 consecutive empty transcripts, allow STT-only mode
when TTS is unavailable.

Knowledge: hydrate IDs immediately after loading more pages so
scroll-based pagination doesn't stall at the bottom.

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

lib/providers/knowledge_provider.dart

@@ -246,6 +246,9 @@ class KnowledgeNotifier extends Notifier<KnowledgeState> {
         isLoadingIds: false,
         hasMore: combined.length < total,
       );
+      // Hydrate the newly fetched IDs immediately — the user is
+      // already at the scroll bottom so _onScroll won't re-fire.
+      await hydrateNext();
     } catch (_) {
       state = state.copyWith(isLoadingIds: false);
     }