fix: Crash-Robustheit + Performance in Backend und Frontend härten

Backend: recover() in allen langlebigen Goroutinen (neues internal/safego-
Paket), MIME-Multipart-Tiefenlimit gegen Stack-Overflow, IMAP-Zeilenlängen-
und FETCH-Result-Limits gegen OOM, MBOX-Buffer-Aliasing-Bug (Datenkorruption
beim Import), ungeprüfte Type Assertions abgesichert, Data Race im
API-Key-Rate-Limiter behoben, SMTP-Session-Panic führt jetzt zu 451-Retry
statt Prozessabsturz. Performance: O(n²)-String-Concat in Mailparser und
IMAP-Parser durch strings.Builder ersetzt.

Frontend: Error Boundaries für Root und Mail-Detailansicht ergänzt (gab es
vorher nicht), zahlreiche Guards gegen nil-Slices aus dem Backend-JSON die
sonst .map()/.length-Crashes/White-Screens auslösten, defekte JSON-Antworten
in api/core.ts abgefangen, zwei React-Key-Bugs bei löschbaren Listen
korrigiert.

Verifiziert auf 192.168.1.132: Build und Tests der geänderten Pakete
fehlerfrei, keine Regressionen gegenüber vorbestehendem Stand.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019j28kGcaJAhBnrYX34hGdt

pkg/mailparser/mbox.go

@@ -3,12 +3,30 @@ package mailparser
 import (
 	"bufio"
 	"bytes"
-	"strings"
+	"fmt"
+)
+
+var (
+	mboxSeparator = []byte("From ")
+	mboxHeaderTag = []byte("From: ")
+	mboxQuoted    = []byte(">From ")
 )
 
 // SplitMbox splits a raw mbox file into individual RFC 2822 message bytes.
 // Each message starts with a "From " separator line which is skipped.
+//
+// Scan errors (e.g. a single line larger than the 10 MB scanner limit) are
+// ignored here for backwards compatibility — use SplitMboxErr to detect a
+// truncated result instead of silently importing partial data.
 func SplitMbox(data []byte) [][]byte {
+	messages, _ := SplitMboxErr(data)
+	return messages
+}
+
+// SplitMboxErr behaves like SplitMbox but also reports scanner failures.
+// On error the already-collected messages are still returned so the caller can
+// decide between aborting and importing a partial file.
+func SplitMboxErr(data []byte) ([][]byte, error) {
 	var messages [][]byte
 	var current bytes.Buffer
 
@@ -17,11 +35,16 @@ func SplitMbox(data []byte) [][]byte {
 
 	inMessage := false
 	for scanner.Scan() {
-		line := scanner.Text()
+		// Bytes() avoids a string allocation per line — mbox files routinely
+		// have millions of lines.
+		line := scanner.Bytes()
 		// mbox separator: line starts with "From " but not "From:" header
-		if strings.HasPrefix(line, "From ") && !strings.HasPrefix(line, "From: ") {
+		if bytes.HasPrefix(line, mboxSeparator) && !bytes.HasPrefix(line, mboxHeaderTag) {
 			if inMessage && current.Len() > 0 {
-				messages = append(messages, bytes.TrimSpace(current.Bytes()))
+				// Copy: current.Bytes() aliases the buffer, which is reused
+				// after Reset() and would corrupt already-collected messages.
+				msg := bytes.TrimSpace(current.Bytes())
+				messages = append(messages, append([]byte(nil), msg...))
 				current.Reset()
 			}
 			inMessage = true
@@ -29,15 +52,20 @@ func SplitMbox(data []byte) [][]byte {
 		}
 		if inMessage {
 			// unescape ">From " lines (mbox quoting)
-			if strings.HasPrefix(line, ">From ") {
+			if bytes.HasPrefix(line, mboxQuoted) {
 				line = line[1:]
 			}
-			current.WriteString(line)
+			current.Write(line)
 			current.WriteByte('\n')
 		}
 	}
 	if inMessage && current.Len() > 0 {
-		messages = append(messages, bytes.TrimSpace(current.Bytes()))
+		msg := bytes.TrimSpace(current.Bytes())
+		messages = append(messages, append([]byte(nil), msg...))
 	}
-	return messages
+
+	if err := scanner.Err(); err != nil {
+		return messages, fmt.Errorf("mailparser: mbox scan aborted after %d messages: %w", len(messages), err)
+	}
+	return messages, nil
 }