Fix panel New Chat hover affordance

[zzj3720]

Summary

• Add a shared chat-header icon hover style/background for panel header controls.
• Apply the hover affordance to the small-window New Chat button and keep adjacent buttons using the same treatment.
• Add accessibility label plus unit/UI coverage for the hover affordance, and document the implementation plan.

Tests

• cd web && yarn install --immutable && yarn build:embedded (passed; bundle size warnings only)
• cd macos && ENABLE_PREVIEWS=YES bash DevKit/Scripts/workspace_test_unit.sh (passed; existing unrelated lint/deprecation warnings)
• cd macos && ENABLE_PREVIEWS=YES xcodebuild test -workspace OpenBridge.xcworkspace -scheme OpenBridge -configuration UnsignedDebug -destination 'platform=macOS' -skipPackagePluginValidation -skipMacroValidation -resultBundlePath .build/TestResults/OpenBridgeUITests-hover.xcresult -only-testing:OpenBridgeUITests/MainWindowUITests/testPanelNewChatButtonHoverShowsVisualFeedback ENABLE_TESTABILITY=YES PRODUCT_BUNDLE_IDENTIFIER=app.openbridge.unsigneddebug PROVISIONING_PROFILE_SPECIFIER='' CODE_SIGN_STYLE=Manual CODE_SIGNING_ALLOWED=YES CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY='-' DEVELOPMENT_TEAM='' (passed)

Manual validation

• Launched the UnsignedDebug app with -e2eMode, hovered the panel New Chat button, and confirmed the visible circular hover ring/tooltip while adjacent header buttons remained intact.