Prevent Mobile Safari from auto-zooming text inputs and textareas

[flmalte]

Summary

• keep real text inputs and textareas at 16px so Mobile Safari does not auto-zoom them on focus
• apply the fix to the shared Phoenix form input component and the custom dashboard search/combobox fields that were still using smaller text sizing
• add a changelog entry for the bugfix

Testing

• git diff --check
• automated tests not run in this environment
• mix is not installed in this environment
• frontend dependencies are not installed in assets/node_modules
• dark mode was not visually verified in this environment

Risk & Rollback

• low risk, this only changes typography sizing for actual text entry fields
• rollback is a straightforward revert of this branch

Linked Issues

• Closes Mobile Safari zooms text inputs below 16px #6235

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