fix: improve screen reader labels in conversations and call screens [WPB-14789] [WPB-14813]

[Garzas]

https://wearezeta.atlassian.net/browse/WPB-14789

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What's new in this PR?

• Marked the chat/call security level icon as decorative for assistive technologies to avoid redundant screen reader output.
• Marked the empty Conversations arrow graphic as decorative so it is hidden from screen readers.
• Updated the New button alternative text to: New. Start a new conversation.
• Removed outdated localized overrides of content_description_new_conversation in selected values-* files so the updated default string is used and can be retranslated in Crowdin.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud