fix: launch PWA at "/" so DEFAULT_HOMEPAGE is respected

[RayMeOut]

Description

Fixes issue #677 by redirecting the start path / to ${DEFAULT_HOMEPAGE} even in a WPA installation. start_url in manifest.json has been changed to the base path /.
Previously, ${DEFAULT_HOMEPAGE} was only taken into account when a user used Split-Pro in a web browser; however, when installed on a smartphone via PWA, the home page was always redirected to /balances (hard-coded in manifest.json). Now, if / is specified as the start URL, the path is redirected to the path defined in the environment variable by the router.

Closes #677

Demo

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Checklist

• I have read CONTRIBUTING.md in its entirety
• I have performed a self-review of my own code
• I have added unit tests to cover my changes
• The last commit successfully passed pre-commit checks
• Any AI code was thoroughly reviewed by me