Add search match highlights to the preview pane

[jongio]

Problem

Search can find the right session, but the preview pane does not show where the query matched. Users have to scroll through long conversations, checkpoints, files, and refs to understand why a result appeared.

Proposed solution

Highlight active search terms in the preview pane and add local next and previous match navigation while the preview is visible. Use the existing search query and preview rendering paths so keyword search still works the same way.

Acceptance criteria

• Active search terms are highlighted in preview metadata, conversation bubbles, checkpoints, files, and refs.
• Highlighting is case-insensitive and works for both quick and deep search results.
• The preview pane shows a match count when matches are present.
• Users can jump to the next and previous match with documented keys that do not conflict with existing global shortcuts.
• Clearing the search query removes highlights and resets match navigation.

Notes

Complexity: M · Priority: 4