Refactor(ui): complete Material icons migration in settings

[insign]

This PR completes the ongoing effort to migrate UI icons from Flutter's default Icons.* to Symbols.* using material_symbols_icons. This ensures visual consistency across the entire application and eliminates the remaining technical debt explicitly noted in CODEBASE.md.

The changes are limited to exactly what was required to finalize this migration, specifically in the behavior_settings_section.dart and notifications_settings_section.dart files. CODEBASE.md was also updated to correctly reflect that this migration task is now finished. No unnecessary dependencies or unrelated changes were introduced.

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PR created automatically by Jules for task 10390768528355914797 started by @insign

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