fix(ui5-date-picker): correct mobile dialog header styling and text

[hinzzx]

Overview

During accessibility testing (ACC-281), several issues were identified with the DatePicker mobile dialog header:

• Wrong font family
• Header text centered instead of left-aligned
• Default header text was "Select" instead of "Pick a date" per VD specs

What We Did

• Correct font family and left-align header text
• By default the text is now "Pick a date" instead of "select"

What This Fixes

• ✅ Mobile dialog header now uses correct font family
• ✅ Header text is left-aligned per VD specs
• ✅ Default header text is "Pick a date" instead of "Select"
• ✅ No functional changes

[ui5-webcomponents-bot]

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