Remove calendar color picker

[Copilot]

Removes the calendar color picker to simplify theming and reduce configuration complexity. The app now uses a fixed default color (#FF9FDF) until theme synchronization is implemented in #7.

Changes:

• Removed palette button from calendar toolbar
• Deleted ColorPickerDialog.razor component
• Removed OpenColorPicker() method from CalendarPage.razor
• Removed ChangePalette test
• Removed PreferenceService.PrimaryColor property and hardcoded the color in GenerateMoodColors()

<issue_title>Remove Calendar color picker</issue_title>
><issue_description>It's hard to guarantee a good-looking theme with the current picker because there are too many choices. Simplify the app now by removing the color picker and using the existing default color (the dark primary), then later we will sync it with the theme in #7.</issue_description>
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> ## Comments on the Issue (you are @copilot in this section)
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• Fixes Remove Calendar color picker #71

Original prompt

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This section details on the original issue you should resolve

<issue_title>Remove Calendar color picker</issue_title>
<issue_description>It's hard to guarantee a good-looking theme with the current picker because there are too many choices. Simplify the app now by removing the color picker and using the existing default color (the dark primary), then later we will sync it with the theme in #7.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Remove Calendar color picker #71

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[danielchalmers]

@copilot Clean up by removing PreferenceService.PrimaryColor too since we only want "#FF9FDF".

[Copilot]

@copilot Clean up by removing PreferenceService.PrimaryColor too since we only want "#FF9FDF".

Done in ed69cdd. Removed the PrimaryColor property and updated GenerateMoodColors() to use the hardcoded color #FF9FDF directly.