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/TranscriptionEditView.svelte

@@ -10,21 +10,12 @@ type Props = {
 	documentId: string;
 	blocks: TranscriptionBlockData[];
 	canComment: boolean;
-	currentUserId: string | null;
 	onBlockFocus: (blockId: string) => void;
 	onSaveBlock: (blockId: string, text: string) => Promise<void>;
 	onDeleteBlock: (blockId: string) => Promise<void>;
 };
 
-let {
-	documentId,
-	blocks,
-	canComment,
-	currentUserId,
-	onBlockFocus,
-	onSaveBlock,
-	onDeleteBlock
-}: Props = $props();
+let { documentId, blocks, canComment, onBlockFocus, onSaveBlock, onDeleteBlock }: Props = $props();
 
 let activeBlockId: string | null = $state(null);
 let saveStates = new SvelteMap<string, SaveState>();
@@ -165,7 +156,6 @@ $effect(() => {
 						active={activeBlockId === block.id}
 						saveState={getSaveState(block.id)}
 						canComment={canComment}
-						currentUserId={currentUserId}
 						onTextChange={(text) => handleTextChange(block.id, text)}
 						onFocus={() => handleFocus(block.id)}
 						onDeleteClick={() => handleDelete(block.id)}