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/routes/documents/[id]/+page.svelte

@@ -9,7 +9,6 @@ let { data } = $props();
 
 const doc = $derived(data.document);
 const canWrite = $derived(data.canWrite ?? false);
-const currentUserId = $derived((data.user?.id as string | undefined) ?? null);
 
 // ── File loading ──────────────────────────────────────────────────────────────
 
@@ -218,7 +217,6 @@ onMount(() => {
 					documentId={doc.id}
 					blocks={transcriptionBlocks}
 					canComment={canWrite}
-					currentUserId={currentUserId}
 					onBlockFocus={handleBlockFocus}
 					onSaveBlock={saveBlock}
 					onDeleteBlock={deleteBlock}