Fix latin1 encoding for German umlauts in bank statements

[Copilot]

FinTS protocol returns bank statement data in latin1 encoding, but the client was decoding as utf-8, causing German umlauts (ä, ö, ü, ß) to display incorrectly.

Changes

• Changed encoding from utf-8 to latin1 in Client.statements() method when parsing booked transactions

// Before
const unprocessedStatements = await read(Buffer.from(bookedString, "utf-8"));

// After
const unprocessedStatements = await read(Buffer.from(bookedString, "latin1"));

Ports the fix from upstream Prior99#34.

Original prompt

Kannst du bitte die Problembehebungen von hier auch übernehmen.

Prior99#34

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[Copilot]

Pull request overview

This PR fixes an encoding issue in the FinTS client where German umlauts (ä, ö, ü, ß) were displaying incorrectly in bank statement data. The fix changes the character encoding from UTF-8 to latin1 (ISO-8859-1) when parsing MT940-formatted bank statements, which is the correct encoding used by the FinTS protocol.

• Changed Buffer.from(bookedString, "utf-8") to Buffer.from(bookedString, "latin1") in the statement parsing logic
• Ports an upstream fix from Prior99#34
• Aligns with existing encoding usage in the codebase (ISO-8859-1 is already used in utils.ts and parse.ts)

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