fix: sort the AP column numerically instead of by its text (#87)

The "AP [minutes / pot%]" column of the station list and of the directed
messages had no comparator, so DataTableState fell back to the rendered
text and "13 (75%)" sorted above "3 (50%)".

AirplaneCell now owns a shared comparator, AirplaneCell.order, that both
column definitions use. The sort key is the first aeroplane of the cell,
which is the one the operator reads and the one the colour marks:
ascending by its arriving minutes, then by the better potential first for
equal minutes. A null info (nothing computed) and an empty list ("nil")
carry Int.MAX_VALUE, so they sort after every real aeroplane in ascending
order, consistent with the qrb and qtf sentinels. The second aeroplane of
a cell does not influence the order. Cell text, colours, column ids,
titles and weights are unchanged, as are DataTableState and the sort
cycle.

Fixes #87

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

github_docs/en-Changelog.md

@@ -40,6 +40,8 @@ Several operators sharing one computer can now use their own callsigns, locators
 
 - **Background resources are released:** the ON4KST supervisor thread, the sked reminder scheduler, the reachability executor, the PSTRotator retry scheduler and the map tile proxy are released when the chat controller is closed. They used to keep running until the program ended.
 
+- **AP column sorts numerically:** the **AP [minutes / pot%]** column in the user list and in the directed messages was sorted by its text, which put `13 (75%)` above `3 (50%)`. It now sorts by the minutes of the first aeroplane and, for equal minutes, by the better potential first; empty and `nil` cells go to the bottom in ascending order. This fixes [Issue #87](https://github.com/praktimarc/kst4contest/issues/87).
+
 ### Upgrade notes
 
 - Existing installations are **not** modified: `preferences.xml` and `praktiKST.db` stay exactly where they are and become the **Default** profile. No file is moved or copied.