feat(frontend): add discussion sub-tab navigation for annotation threads

Within the Diskussion panel tab, show two sub-tabs when an annotation is
active: «Diskussion» (document-level thread, with comment-count badge) and
«Annotation · Seite N» (annotation-specific thread).

Behaviour:
- Clicking an annotation auto-switches to the Annotation sub-tab
- Clicking the Diskussion sub-tab deselects the annotation and returns to
  the document thread
- Escape clears the active annotation (or collapses the panel if none)
- activeAnnotationPage is now lifted from PdfViewer → DocumentViewer →
  page → DocumentBottomPanel → PanelDiscussion so the tab label shows the
  correct page number

Closes #60
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/es.json

@@ -257,5 +257,6 @@
 	"doc_panel_tab_history": "Historial",
 	"doc_panel_annotate": "Anotar",
 	"doc_panel_annotate_stop": "Listo",
-	"doc_panel_annotation_thread_title": "Anotación"
+	"doc_panel_annotation_thread_title": "Anotación",
+	"doc_panel_discussion_annotation_tab": "Anotación · Página {page}"
 }