Optimize action/plugin search caching with normalized CachedSearchEntry

[multiplex55]

Motivation

• Avoid repeated to_lowercase() and temporary string allocations during search and exact-match filtering by pre-normalizing searchable fields.
• Make plugin command cache refresh deterministic and tied to existing reload paths so cached search entries stay consistent after plugin/settings changes.
• Improve search correctness by including normalized action id in exact-mode filtering alongside label/desc/aliases.

Description

• Added a CachedSearchEntry struct that stores label_lc, desc_lc, and action_lc, and replaced the old action_cache: Vec<(String,String)> with action_cache: Vec<CachedSearchEntry> and added command_search_cache: Vec<CachedSearchEntry> in src/gui/mod.rs.
• Updated update_action_cache and update_command_cache to populate normalized cache entries via CachedSearchEntry::from_action so normalization happens once per reload.
• Refactored matches_exact_display_text, search_actions, and search_plugins to consume pre-normalized CachedSearchEntry data and to check action_lc as an additional exact-match target, while preserving existing plugin-resolved bypass logic.
• Ensured update_paths calls update_command_cache unconditionally so plugin/settings reloads refresh the command_search_cache and keep caches in sync.

Testing

• Added unit tests: exact_display_match_is_case_insensitive_substring, action_and_command_search_cache_is_normalized, and update_paths_refreshes_command_search_cache_for_plugin_reload in src/gui/mod.rs to validate normalized caching and behavior.
• Ran cargo fmt --all, which completed successfully.
• Attempted to run cargo test gui::mod::tests:: -- --nocapture, but the test build failed in this environment due to a native dependency error from alsa-sys requiring system alsa/pkg-config (not available here), so full test execution could not complete.
• cargo fmt --all --check was used to ensure formatting; repository formatting differences unrelated to this change were noted during the rollout and addressed by running cargo fmt.

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Codex Task