Fix ASSA Title field not being saved in properties dialog

[niksedk]

Problem

Reported in #11634: in the ASSA (Advanced SubStation Alpha) Properties dialog, the Title field is shown and editable, but the edited value is not saved.

Cause

AssaPropertiesViewModel.LoadFromHeader() reads Title: from the [Script Info] header into ScriptTitle, but UpdateHeader() (run on OK) wrote back every other Script Info property except Title, so the value was silently discarded.

The format layer (AdvancedSubStationAlpha.cs) round-trips Title: correctly — it writes subtitle.Header verbatim — so the value just never made it back into the header from the UI.

Fix

Add the missing call in UpdateHeader():

UpdateTag("Title", ScriptTitle, string.IsNullOrWhiteSpace(ScriptTitle));

This matches the SSA properties dialog (SsaPropertiesViewModel.cs), which already has this line and works correctly. No change is needed for SSA.

Notes

• Only the ASSA dialog was affected; SSA already handled Title.
• The unrelated IME/input-method issue also mentioned in The Input Method Editor is not working, the Title field is not saved in ass format #11634 is a separate problem and is not addressed here.

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