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bug: Built application fails when paths or TOML configuration contain German umlaut characters #6641

Description

@sschroedel

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Describe the bug

The built application does not correctly handle German umlaut characters such as ä, ö, ü, Ä, Ö, Ü, and ß.

After building the application, any file path, directory path, or other path-related value containing an umlaut causes the application to fail or become unusable. The same issue occurs in TOML configuration files: if any value in the TOML file contains an umlaut character, the application fail to start with an unrelated error.

This appears to be a character encoding or Unicode handling issue in the built application.

This is a significant compatibility issue for users in German-speaking environments. Umlaut characters are common in:

Usernames
Windows user profile directories
Folder and file names
Project names
Configuration values

Users whose system paths or configuration files contain these characters may be unable to run the application.

Code sample

Code
[Paste your code here]

To reproduce

Steps to Reproduce
Scenario 1: Path contains an umlaut
Build the application.
Place or run the application in a directory whose path contains a German umlaut.

Example:

C:\Users\Jürgen\Documents\Application

or

C:\Projekte\Überprüfung\App

Start the application.
Scenario 2: TOML file contains an umlaut
Build the application.
Add a German umlaut character to any relevant value in the TOML configuration file.

Example:

name = "Überprüfung"
path = "C:\Users\Jürgen\Documents"
Start the application.
Actual Result

The application fails to run or becomes unusable when:

A file or directory path contains a German umlaut character.
A value in the TOML configuration contains a German umlaut character.
Expected Result

The application should fully support Unicode characters, including German umlauts, in:

File paths
Directory paths
User profile paths
Configuration values
TOML files

The application should start and operate normally regardless of whether these characters are present.

Expected behavior

The built application starts successfully from paths containing ä, ö, ü, Ä, Ö, Ü, and ß.
File and directory paths containing Unicode characters are handled correctly.
TOML configuration files containing German umlauts are parsed successfully.
Configuration values containing Unicode characters do not prevent application startup.
Automated tests are added to cover Unicode characters in both paths and TOML configuration values.
The fix is verified in the packaged/built application, not only in the development environment.
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Operating System

Windows

Operating system details

Winn 11 25H2

Flet version

Flet 0.85.2 on Windows 11 (AMD64) Python 3.14.0

Regression

No, it isn't

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