feat(frontend): add OCR UI components and translations

- ScriptTypeSelect: native select for TYPEWRITER/HANDWRITING_LATIN/KURRENT
- OcrTrigger: wraps script type select + start button + confirmation dialog
- OcrProgress: SSE-based progress display with page counter and progress bar
- Paraglide translations for OCR (de/en/es): script types, trigger labels,
  confirmation dialog, progress messages, error messages
- ErrorCode type + getErrorMessage: OCR_SERVICE_UNAVAILABLE, OCR_JOB_NOT_FOUND,
  OCR_DOCUMENT_NOT_UPLOADED, OCR_PROCESSING_FAILED

All 687 frontend tests pass.

Refs #226

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

frontend/src/lib/components/OcrTrigger.svelte

@@ -0,0 +1,49 @@
+<script lang="ts">
+import { untrack } from 'svelte';
+import * as m from '$lib/paraglide/messages.js';
+import { getConfirmService } from '$lib/services/confirm.svelte';
+import ScriptTypeSelect from './ScriptTypeSelect.svelte';
+
+interface Props {
+	existingBlockCount: number;
+	storedScriptType: string;
+	onTrigger: (scriptType: string) => void;
+}
+
+let { existingBlockCount, storedScriptType, onTrigger }: Props = $props();
+
+const { confirm } = getConfirmService();
+
+let selectedScriptType: string = $state(
+	untrack(() => (storedScriptType && storedScriptType !== 'UNKNOWN' ? storedScriptType : ''))
+);
+
+async function handleClick() {
+	if (!selectedScriptType) return;
+
+	if (existingBlockCount > 0) {
+		const confirmed = await confirm({
+			title: m.ocr_confirm_title(),
+			body: m.ocr_confirm_body({ count: String(existingBlockCount) }),
+			confirmLabel: m.ocr_confirm_btn(),
+			destructive: true
+		});
+		if (!confirmed) return;
+	}
+
+	onTrigger(selectedScriptType);
+}
+</script>
+
+<div class="flex flex-col gap-3">
+	<ScriptTypeSelect bind:value={selectedScriptType} />
+	<button
+		type="button"
+		disabled={!selectedScriptType}
+		title={!selectedScriptType ? m.ocr_trigger_btn_disabled() : undefined}
+		onclick={handleClick}
+		class="min-h-[44px] w-full rounded-sm bg-brand-navy font-sans text-sm font-medium text-white transition-colors hover:bg-brand-navy/90 disabled:cursor-not-allowed disabled:opacity-50"
+	>
+		{m.ocr_trigger_btn()}
+	</button>
+</div>