Feat: Add Advanced Formula-Based Search to Friend Feed

[rnmmr]

Explain in detail what your suggested feature would be used for.

This pull request adds support for advanced formula-based search to the Friend Feed, enabling users to execute powerful and flexible queries using a formula syntax (e.g., =name=Alice & type=GPS). The change is adapted from rnmmr@986d547 and includes:

• Integration of a new search formula parser and evaluator (src/shared/utils/feedSearchFormula.js) with comprehensive unit tests
• Minimal UI additions: Limited to a help dialog and search icon in the Feed view
• Full i18n support for help documentation and error handling
• Improved error reporting and feedback for search input
• Enhanced backend feed search algorithm with date/time extraction and field-specific logic
• User search support: Added usr_xxx prefix to search all feed records for a specific user
• Search Depth control: Users can adjust search depth directly from the Feed filter popover
• Performance optimizations: Implemented efficient query merging and keyword extraction; designed to handle large datasets (1000-4000 friends, 8GB+ databases) per VRCX performance baseline
• Safety features: Added input validation, robust error handling, and graceful recovery
• Time-based filtering: Support for year/month/day/hour/minute/second granularity time range queries
• Non-disruptive opt-in: Formula search activates only with = prefix; native keyword search remains fully functional

Describe how it would look if it requires a UI.

• Minimal UI changes: The core functionality is implemented in backend utilities
• Adds a search help icon and dialog to the Feed page explaining formula syntax
• Search bar accepts formula (starting with =) and provides error feedback
• Help documentation localized in English and Chinese with usage examples
• No changes to existing layouts or styling - only adds documentation elements
• Backward compatible: Native keyword search works exactly as before

Explain why people would want to use it.

• Broadly applicable: Enables precise filtering for users with large friend lists (1000-4000+ friends) who need advanced auditing capabilities
• Powerful queries: Search friend feed history with complex combinations (names, events, dates, types, etc.)
• No disruption: Users can continue using simple keyword search; formula search is completely optional
• Lower learning curve: Multi-language help with practical examples makes it accessible to new users
• Maintenance friendly: Well-tested with 65+ unit tests, minimal UI footprint reduces future maintenance burden

Usage Examples:

• =name=Alice & type=GPS - Search GPS updates for Alice
• =time>2024/04 & type!=Offline - Search non-offline events in April
• =name=[Alice,Bob,Charlie] - Search records for multiple users
• =world="Test World" & time=2024/04/01~2024/04/30 - Search records in specific world and date range
• =type!=GPS - Exclude GPS type records