feat(search): in-chat message search and session list filter

[MassimilianoPili]

Summary

Two client-side search capabilities, mirroring the existing file/command search UX (transparent-bordered field, TUI glyphs):

• In-chat message search — searches the open conversation across text and reasoning parts, with a live match counter (3/12), previous/next navigation and scroll-to-match plus a subtle highlight on the current hit. Opened from the chat overflow menu (⌕ Search chat); Back closes the search bar.
• Session-list filter — a search field toggled from the sessions top bar that filters the conversation list by title. While searching, the tree flattens to matching sessions so a child session isn't hidden by a filtered-out parent.

Implementation

• ChatSearch.kt — pure, testable match logic over MessageBlocks (Part.Text/Part.Reasoning/tool names/subtask prompts).
• ChatSearchBar.kt — the in-chat search bar composable.
• ChatScreen.kt — search state, scroll-to-match, current-hit highlight via itemsIndexed, Back handler.
• SessionListScreen.kt — sticky-header search field + title filter.
• New string resources only; no dependency or build config changes.

Testing

Built with JDK 17 (./gradlew :app:assembleDebug) and verified on a physical device: both searches work, match navigation scrolls correctly, no crashes.

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[ghost]

Ready to review this PR? Stage has broken it down into 5 individual chapters for you:

	Title
1	Define search-related string resources
2	Implement core chat search logic
3	Create the in-chat search bar UI
4	Integrate search into the chat screen
5	Implement session list filtering

_{Chapters generated by Stage for commit f9a3491 on Jun 21, 2026 7:34pm UTC.}