Stop Account Name and Username refilling after they are cleared

[Gabriel-Dalton]

Fixes #451.

Problem

In Add Account, clearing Account Name (or Username) makes it fill itself straight back in, so the field cannot be emptied or retyped from scratch.

The auto-fill $effect in AccountForm.svelte reads name and username to check whether they are empty. In Svelte 5 that makes them dependencies of the effect, so clearing either one re-runs it and it refills the field.

Change

• Read name, username and prevEmail with untrack, so editing those fields never re-runs the auto-fill. It still runs when the email changes.
• Track the name the form filled in (prevAutoName) and only overwrite a name that is empty or still equal to it. A name the user typed is never replaced.

Side effect: the name now follows the email while it is being typed. Before, typing the address one character at a time left the name stuck at the first letter, because the field was no longer empty after the first keystroke.

Edit mode is unaffected: the loaded account name differs from prevAutoName, so it is kept.

Checks

• svelte-check: 0 errors, 0 warnings
• eslint on the file: clean
• wails build on Windows 11: builds