feat(filename): support compound last names like de Gruyter

Replace the four fixed regexes with a split-based algorithm:
- first segment = date → last segment = firstName, rest = lastName parts
- last segment = date → second-to-last = firstName, rest = lastName parts

18881025_de_Gruyter_Walter.pdf now correctly yields "Walter de Gruyter".
Simple two-segment names behave identically to before.

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

frontend/src/lib/utils/filename.ts

@@ -9,46 +9,73 @@ export interface FilenameParseResult {
 	suggestedTitle?: string;
 }
 
-// Full-match patterns only. Name segments use Unicode letters (\p{L}) to cover umlauts etc.
-// Order: date_lastname_firstname
-const P_DATE_ISO_NAME = /^(\d{4}-\d{2}-\d{2})_(\p{L}+)_(\p{L}+)\.[^.]+$/u;
-const P_DATE_COMPACT_NAME = /^(\d{8})_(\p{L}+)_(\p{L}+)\.[^.]+$/u;
-// Order: lastname_firstname_date
-const P_NAME_DATE_ISO = /^(\p{L}+)_(\p{L}+)_(\d{4}-\d{2}-\d{2})\.[^.]+$/u;
-const P_NAME_DATE_COMPACT = /^(\p{L}+)_(\p{L}+)_(\d{8})\.[^.]+$/u;
-
-function compactToIso(compact: string): string {
-	return `${compact.slice(0, 4)}-${compact.slice(4, 6)}-${compact.slice(6, 8)}`;
+// A date token is either YYYY-MM-DD or YYYYMMDD with a plausible month/day range.
+function tryParseDate(s: string): string | undefined {
+	if (/^\d{4}-\d{2}-\d{2}$/.test(s)) {
+		const m = parseInt(s.slice(5, 7));
+		const d = parseInt(s.slice(8, 10));
+		if (m >= 1 && m <= 12 && d >= 1 && d <= 31) return s;
+	} else if (/^\d{8}$/.test(s)) {
+		const m = parseInt(s.slice(4, 6));
+		const d = parseInt(s.slice(6, 8));
+		if (m >= 1 && m <= 12 && d >= 1 && d <= 31)
+			return `${s.slice(0, 4)}-${s.slice(4, 6)}-${s.slice(6, 8)}`;
+	}
+	return undefined;
 }
 
+const NAME_PART = /^\p{L}+$/u;
+
+/**
+ * Parses a structured filename and extracts a date and person name.
+ *
+ * Supported conventions (date-first or date-last, compound last names supported):
+ *   YYYY-MM-DD_Lastname_Firstname.ext
+ *   YYYYMMDD_Lastname_Firstname.ext
+ *   YYYYMMDD_de_Gruyter_Walter.ext   ← compound last name: lastName="de Gruyter"
+ *   Lastname_Firstname_YYYY-MM-DD.ext
+ *   Lastname_Firstname_YYYYMMDD.ext
+ *   de_Gruyter_Walter_YYYYMMDD.ext   ← compound last name: lastName="de Gruyter"
+ *
+ * Algorithm: split on "_", identify the date token (first or last segment),
+ * treat the outermost remaining segment as firstName, rest as lastName parts.
+ * Returns {} for anything that doesn't match cleanly.
+ */
 export function parseFilename(filename: string): FilenameParseResult {
+	const dot = filename.lastIndexOf('.');
+	if (dot < 0) return {}; // no extension — not a real file
+	const stem = filename.slice(0, dot);
+	const parts = stem.split('_');
+
+	// Minimum: date + at least one lastName segment + firstName = 3 parts
+	if (parts.length < 3) return {};
+
 	let dateIso: string;
-	let lastName: string;
-	let firstName: string;
+	let nameParts: string[];
 
-	let m: RegExpMatchArray | null;
-
-	if ((m = P_DATE_ISO_NAME.exec(filename))) {
-		[, dateIso, lastName, firstName] = m;
-	} else if ((m = P_DATE_COMPACT_NAME.exec(filename))) {
-		dateIso = compactToIso(m[1]);
-		lastName = m[2];
-		firstName = m[3];
-	} else if ((m = P_NAME_DATE_ISO.exec(filename))) {
-		lastName = m[1];
-		firstName = m[2];
-		dateIso = m[3];
-	} else if ((m = P_NAME_DATE_COMPACT.exec(filename))) {
-		lastName = m[1];
-		firstName = m[2];
-		dateIso = compactToIso(m[3]);
+	const dateFromFirst = tryParseDate(parts[0]);
+	if (dateFromFirst) {
+		dateIso = dateFromFirst;
+		nameParts = parts.slice(1);
 	} else {
-		return {};
+		const dateFromLast = tryParseDate(parts[parts.length - 1]);
+		if (!dateFromLast) return {};
+		dateIso = dateFromLast;
+		nameParts = parts.slice(0, -1);
 	}
 
+	// Need at least lastName + firstName after removing the date
+	if (nameParts.length < 2) return {};
+
+	// All name segments must be pure letters (covers umlauts via \p{L})
+	if (!nameParts.every((p) => NAME_PART.test(p))) return {};
+
+	const firstName = nameParts[nameParts.length - 1];
+	const lastName = nameParts.slice(0, -1).join(' ');
 	const personName = `${firstName} ${lastName}`;
-	const suggestedTitle = `${personName} (${isoToGerman(dateIso!)})`;
-	return { dateIso: dateIso!, personName, suggestedTitle };
+	const suggestedTitle = `${personName} (${isoToGerman(dateIso)})`;
+
+	return { dateIso, personName, suggestedTitle };
 }
 
 export function stripExtension(filename: string): string {