Navigation skips a month if the selected day does not exist in it

### What Happened?

Screen recording showing how it jumps from January 30th to March 2nd, skipping February because it doesn't have a 30th day.

https://github.com/user-attachments/assets/7beb9123-82c8-4ff1-9f7d-7f8cf671a8f9

While it may be logical to skip it, it's disorientating.

### Steps to Reproduce

1. Open the Calendar and switch to January.
2. Click on one of its last days (e.g. 30th) to select it.
3. Switch to the next month (by scrolling or using the navigation buttons at the headerbar).

### Expected Behavior

I would prefer to have it select the last day of that month (e.g. February 28th, 2026) instead of skipping it.

### OS Version

8.x (Circe)

### OS Architecture

amd64 (on most hardwares)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Latest release (I have run all updates)

### Log Output

```shell

```

### Hardware Info

_No response_

Start by reproducing the issue in the Calendar: select January 30th, then move to the next month with scrolling or the headerbar navigation buttons. Trace the month-change and selected-day handling, then verify that moving to a shorter month selects its final day, such as February 28th, instead of skipping ahead.

Written by the indexing model from the issue text.