refactor(comments): flat compact comment thread matching spec design

Rework CommentThread.svelte to match the annotation-transcription spec:
- Flat message list (no nested reply threading)
- Compact inline style: orange left border, tinted background
- Chat bubble icon (💬) with comment count header
- Avatar circles with author initials
- Quoted text extracted and rendered as italic left-bordered snippet
- Simple MentionEditor input at bottom (keeps @mention support)
- Removed: reply-to-specific threading, edit/delete buttons, nesting

Remove dead components no longer used after annotate mode removal:
- AnnotationCommentPanel, AnnotationSidePanel, AnnotateHintStrip
- PanelDiscussion, PanelHistory, PanelMetadata, PanelTranscription
- Associated spec files

Simplify prop chain: remove currentUserId, canAdmin, targetCommentId
from CommentThread, TranscriptionBlock, TranscriptionEditView.

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

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -13,7 +13,6 @@ type Props = {
 	active: boolean;
 	saveState: SaveState;
 	canComment: boolean;
-	currentUserId: string | null;
 	onTextChange: (text: string) => void;
 	onFocus: () => void;
 	onDeleteClick: () => void;
@@ -29,7 +28,6 @@ let {
 	active,
 	saveState,
 	canComment,
-	currentUserId,
 	onTextChange,
 	onFocus,
 	onDeleteClick,
@@ -204,8 +202,6 @@ function captureSelectionAndOpenComments() {
 				blockId={blockId}
 				loadOnMount={true}
 				canComment={canComment}
-				currentUserId={currentUserId}
-				canAdmin={false}
 				quotedText={selectedQuote}
 				showCompose={commentOpen}
 			/>