feat(frontend): add floating bottom panel to document detail page

Replaces the left sidebar layout with:
- Full-viewport PDF/image viewer (never resizes, position: absolute)
- Fixed floating bottom panel with tabs: Metadaten, Transkription,
  Diskussion, Verlauf
- Compact top bar with title, date · sender → receivers row, and
  Annotieren / Edit / Download actions
- Drag-to-resize panel with localStorage persistence of open/height/tab
- Panel opens automatically to Diskussion when an annotation is clicked
- Documents without a file default to showing the Metadaten tab

New components: DocumentTopBar, DocumentViewer, DocumentBottomPanel,
PanelMetadata, PanelTranscription, PanelDiscussion, PanelHistory

PdfViewer: annotateMode and activeAnnotationId lifted to bindable props;
AnnotationCommentPanel removed (discussion moves to the Diskussion tab).

Closes #62
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -250,5 +250,12 @@
 	"comment_btn_reply": "Antworten",
 	"comment_edited_label": "· bearbeitet",
 	"comment_panel_title": "Kommentare",
-	"comment_panel_close": "Schließen"
+	"comment_panel_close": "Schließen",
+	"doc_panel_tab_metadata": "Metadaten",
+	"doc_panel_tab_transcription": "Transkription",
+	"doc_panel_tab_discussion": "Diskussion",
+	"doc_panel_tab_history": "Verlauf",
+	"doc_panel_annotate": "Annotieren",
+	"doc_panel_annotate_stop": "Fertig",
+	"doc_panel_annotation_thread_title": "Annotation"
 }