fix: address PR review feedback — security, architecture, dead code

Fixes from PR #178 review:

Migration fixes:
- V18/V19: fix FK references from app_users to users (correct table name)
- V18: change annotation_id FK from ON DELETE CASCADE to ON DELETE RESTRICT
  (block is aggregate root, cascade flows from block, not annotation)

Backend fixes:
- TranscriptionService.deleteBlock(): remove userId param, delete block first
  then annotation directly via repository (no ownership check — block owns annotation)
- TranscriptionService.sanitizeText(): remove flawed regex HTML stripping,
  textarea content is plain text by design — just enforce max length
- TranscriptionBlockController.requireUserId(): throw DomainException.unauthorized()
  instead of silently returning null on auth failure
- CreateTranscriptionBlockDTO: add @Min/@Positive validation on coordinates
- Add @Slf4j logging to TranscriptionService for create/delete operations

Frontend fixes:
- Delete DocumentBottomPanel.svelte entirely (issue #175 requirement)
- Remove redundant mode exclusivity $effect (handled at toggle call sites)
- Remove dead handleCommentClick + onCommentClick prop (comments are future work)
- Remove quote hint UI (depends on comment feature)

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

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -12,7 +12,6 @@ type Props = {
 	saveState: SaveState;
 	onTextChange: (text: string) => void;
 	onFocus: () => void;
-	onCommentClick: () => void;
 	onDeleteClick: () => void;
 	onRetry: () => void;
 };
@@ -26,7 +25,6 @@ let {
 	saveState,
 	onTextChange,
 	onFocus,
-	onCommentClick,
 	onDeleteClick,
 	onRetry
 }: Props = $props();
@@ -91,22 +89,7 @@ function handleDelete() {
 		></textarea>
 
 		<!-- Footer -->
-		<div class="flex items-center justify-between border-t border-line pt-2">
-			<div class="flex flex-col gap-1">
-				<button
-					type="button"
-					class="text-xs font-medium text-ink-2 transition-colors hover:text-ink"
-					onclick={onCommentClick}
-				>
-					{m.transcription_block_comment_btn()}
-				</button>
-				{#if active}
-					<span class="text-xs text-ink-3">
-						{m.transcription_block_quote_hint()}
-					</span>
-				{/if}
-			</div>
-
+		<div class="flex items-center justify-end border-t border-line pt-2">
 			<div class="flex items-center gap-2">
 				<!-- Save state indicator -->
 				{#if saveState === 'saving'}