fix(GH-1782): harden parseJwt validation and error handling

[ThomasAFink]

Validate JWT structure, apply base64url padding before atob, and throw JwtParseError with clear messages when decoding or JSON parsing fails.

Pull Request Checklist (Feature Branch to next):

• Ich habe die neuesten Änderungen aus dem next Branch in meinen Feature-Branch gemergt.
• Relevante Tests wurden mit zmsautomation ausgeführt.
• Das Code-Review wurde abgeschlossen.
• Fachliche Tests wurden durchgeführt und sind abgeschlossen.
• Ich habe erforderliche Dokumentation im Ordner docs hinzugefügt.

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📒 Files selected for processing (2)

• zmscitizenview/src/utils/auth.ts
• zmscitizenview/tests/unit/utils/auth.spec.ts

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[ThomasAFink]

zmsautomation: https://github.com/it-at-m/eappointment/actions/runs/26103654526