Fix hardcoded paths with environment variable support

[xingzihai]

Summary

This PR fixes hardcoded path issues throughout the codebase by implementing a centralized path configuration system with environment variable support.

Changes

Core Path Configuration (scripts/lib/env.py)

• Added centralized path functions that support environment variable overrides
• Implemented XDG Base Directory specification compliance
• Added fallback to temp directories when permission denied

Environment Variables Supported

Variable	Default	Description
LAST30DAYS_CONFIG_DIR	~/.config/last30days	Config directory
LAST30DAYS_DATA_DIR	~/.local/share/last30days	Data directory for database
LAST30DAYS_DATABASE_PATH	{DATA_DIR}/research.db	Full database path
LAST30DAYS_CACHE_DIR	~/.cache/last30days	Cache directory
LAST30DAYS_OUTPUT_DIR	~/.local/share/last30days/out	Output directory
LAST30DAYS_BRIEFS_DIR	~/.local/share/last30days/briefs	Briefings directory

Files Modified

• scripts/lib/env.py - Added path configuration functions
• scripts/store.py - Updated to use get_database_path()
• scripts/lib/cache.py - Updated to use get_cache_dir()
• scripts/lib/render.py - Updated to use get_output_dir()
• scripts/last30days.py - Updated help string for --store
• scripts/lib/ui.py - Updated to dynamically show config paths
• scripts/briefing.py - Updated to use get_briefs_dir()
• README.md - Added path customization documentation

Benefits

1. Portability - Users can now customize storage locations for different environments (containers, shared systems, etc.)
2. XDG Compliance - Follows the XDG Base Directory specification
3. Graceful Fallback - Automatically falls back to temp directories if permission denied
4. Backward Compatible - All existing defaults are preserved

Testing

All modified Python files compile successfully without errors.

[mvanhorn]

See my reply on #109 covering all your PRs. Can't commit to anything right now with the v3.0 refactor underway, but I promise I'll consider each one once it lands. Appreciate the work.