[WIP] Fix average calculation based on days with entries

[Copilot]

Plan to fix average calculations

• Explore repository structure and understand the issue
• Identify the problem: averages are calculated based on days with entries, not calendar days
• Update computeAverage function in statistics-utils.dart to:
  • For past periods: count all calendar days from start date to today (or end date if earlier)
  • For future periods: only count days that have entries
• Update average calculation in overview-card.dart to use the same logic
• Create tests for the new average calculation logic
• Run tests to validate changes
• Verify the fix works as expected

Original prompt

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This section details on the original issue you should resolve

<issue_title>Average calculation</issue_title>
<issue_description>Hi,

the averages are calculated based on the number of days with entries, not calendar days in the month/year.
I guess this is not on purpose, as it is not very helpful!? 🙂

Great app btw
Thanks
DrDee</issue_description>

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

@emavgl I don't see it very helpful neither if it would count the day with 0 expenses, as missing entries are not the same of spending 0 money. But I get your point here. Do you find the opposite helpful? @emavgl I don't have a hard opinion, so if you find it useful, I'll adapt it the way you said:

• Count the day with no expenses in the past, but not in the future</comment_new>
  <comment_new>@emavgl
  Well those should count I believe. As the user has inserted the value in the future, he would probably be interested in seeing that</comment_new>

• Fixes Average calculation #310

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