Suppress autocomplete suggestions when dashboard is visible

[multiplex55]

Motivation

• Avoid showing autocomplete suggestions when the dashboard should be displayed to prevent stale or confusing UI state.
• Keep dashboard visibility logic consistent by reusing should_show_dashboard across suggestion updates and rendering.
• Ensure suggestions are cleared on empty query or when entering dashboard mode so stale entries do not persist.
• Prevent rendering suggestions underneath the dashboard UI.

Description

• Updated update_suggestions() to return early if !self.query_autocomplete, self.query.is_empty(), or self.should_show_dashboard(self.query.as_str()).
• In the main UI rendering path compute use_dashboard = self.should_show_dashboard(trimmed.as_str()) and use it to skip drawing suggestions (self.query_autocomplete && !use_dashboard && !self.suggestions.is_empty()).
• Clear self.suggestions and reset self.autocomplete_index when entering the empty-query case and when switching into dashboard mode.
• Changes are localized to src/gui/mod.rs and reuse the existing should_show_dashboard helper for consistent behavior.

Testing

• No automated tests were run.

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Codex Task