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.spec.ts

@@ -15,7 +15,6 @@ function renderBlock(overrides: Record<string, unknown> = {}) {
 		active: false,
 		saveState: 'idle' as const,
 		canComment: true,
-		currentUserId: 'user-1',
 		onTextChange: vi.fn(),
 		onFocus: vi.fn(),
 		onDeleteClick: vi.fn(),