#### Brief overview of PR changes/additions
Restores the ability for the connection dialog to load saved passwords
from the portable password file on disk, which stopped working after PR
#7956 introduced secure credential management.
#### Motivation for adding to Mudlet
After #7956, users who had passwords saved in the old portable file
format (`~/.config/mudlet/profiles/<name>/password`) could no longer see
their passwords pre-filled when selecting a profile in the connection
dialog. This affected users who had not yet migrated to the system
keychain.
The fix ensures all three password sources are checked in order: system
keychain, QSettings, and finally the portable file on disk. It also
moves the password migration step to run before the connection dialog
opens, so passwords are migrated in time to be found.
Includes the profile password collision fix from #8948 and adds new unit
tests for the password migration and fallback paths.
#### Other info (issues closed, discussion etc)
Reported by @SlySven — PR #7956 (commit 79ab7be4) broke password loading
for profiles that only had passwords in the portable file.
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Co-authored-by: Stephen Lyons <slysven@virginmedia.com>
