feat: force account picker when adding a Spotify account

[felixschndr]

Proposed change

Send show_dialog=true on the authorize URL so Spotify shows the approval screen and the user can pick a different account.

Also abort account creation flow when account is already setup. This lead to the log message 2026-07-30 17:22:53.999 WARNING (MainThread) [homeassistant.helpers.frame] Detected that integration 'spotify' creates a config entry when another entry with the same unique ID exists. This will stop working in Home Assistant 2026.3, please create a bug report at https://github.com/home-assistant/core/issues?q=is%3Aopen+is%3Aissue+label%3A%22integration%3A+spotify%22.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
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• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
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Additional information

• This PR fixes or closes issue: fixes Spotify does not find linked accounts #177665
• This PR is related to issue:
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[home-assistant]

Hey there @frenck, @joostlek, mind taking a look at this pull request as it has been labeled with an integration (spotify) you are listed as a code owner for? Thanks!

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