feat(gui): hold preview while space is pressed

[StudentWeis]

Summary

Changes Space preview from toggle mode to hold-to-preview mode, and adds a dedicated Space Preview setting so users can disable that shortcut independently of hover preview.

Linked Issue

Closes #111

Changes

• Show preview on Space key down and hide it on Space key up, while ignoring the shortcut when search input or the clear-confirm dialog owns focus.
• Add a persisted space_preview_enabled setting with defaults and save/load coverage.
• Expose a new Space Preview switch in the Settings panel and reset its state from persisted config.
• Update localized settings/help copy to describe the hold-to-preview behavior.
• Add tests for hold-only preview visibility and the disabled-setting path.

Testing

• scripts/precheck.sh passes locally
• New / updated tests cover the change (test_preview_state_is_visible_only_while_space_is_held, test_space_preview_setting_blocks_space_preview_when_disabled, test_preview_settings_default)

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — i18n keys added to all locale files
• New UI components use gpui-component (existing Switch reused; no new component types)
• Errors defined with thiserror (no new error types)
• No unrelated changes mixed in