feat: message search within conversation

[sakebomb]

Summary

• Adds in-conversation message search with real-time filtering and text highlighting
• Search bar appears via toolbar icon with auto-focus, match count indicator, and close button
• Matching text highlighted using <mark> tags via SearchQueryContext (avoids prop drilling through ChatView → Message → TextMessage → MarkdownView chain)

Implementation details

• ChatSessionStore: isSearchMode, searchQuery, enterSearchMode(), exitSearchMode(), setSearchQuery(), filteredSessionMessages getter (case-insensitive text filtering)
• ChatSearchBar: New component with search input, match count/no-results indicator, close button
• HeaderRight: Search icon toggle button (visible when session is active)
• ChatScreen: Passes filtered messages and search bar via customContent prop; wraps ChatView in SearchQueryContext.Provider
• MarkdownView: Consumes SearchQueryContext, wraps matching text in <mark> tags after markdown-to-HTML conversion, registers mark element model with react-native-render-html
• l10n: chat.searchMessages, chat.noResults strings added to en.json

Test plan

• 8 new store tests: enter/exit search mode, query setting, filtering, case-insensitive matching, non-text message exclusion
• 7 new ChatSearchBar component tests: rendering, close button, text change handler, match count display, no results display
• 2 new MarkdownView tests: rendering with/without SearchQueryContext
• All 137 existing tests continue to pass
• TypeScript compiles cleanly
• ESLint passes (no new warnings)

Closes #603