Add the option to sort read books by the date the book was finished

### Feature Request

Currently, it's only possible to view the list of our read books sorted by the date they were added (most recent/oldest, ascending/descending). It would be great to also be able to view them sorted by the date they were read (finished). This way, if we add now a book that we read a long time ago, that book can still appear correctly ordered on our list, instead of at the top.

Since it's possible to add a date without having to fill in all the fields (just the year, or just the year and the month) --a very useful feature that should be maintained, because we don't always remember the exact date we finished a book--, a solution to this, if technically possible, could be to set the read order based on the total number of fields filled in.

Thus, for a descending order, the read (date finished) order could be as follows:

YY
YY-MM
YYYY-MM-DD

Example:

Descending:
2025
2025-10
2025-10-07
2024
2024-10
2024-10-07

Ascending:
2024-10-07
2024-10
2024
2025-10-07
2025-10
2025

Thank you.

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The issue names no files, tests, or entry points. Start by tracing how the read-books list currently applies added-date sorting and how partially specified finished dates are stored. Done means users can sort read books by finished date in both directions while preserving support for year-only and year-month dates.

Written by the indexing model from the issue text.