fix: prevent duplicate recent files and remove non-existent entries

[robcohen]

Problem

• Recent files could accumulate duplicates when paths had different representations (e.g., /foo/./bar vs /foo/bar)
• Deleted files remained in the recent list

Solution

• Normalize paths via canonicalize() before storing
• Filter non-existent files when rendering the list
• Auto-clean storage when stale entries are found

New Tauri commands

• normalize_path(path) - Returns canonical path or error if file doesn't exist
• filter_existing_paths(paths) - Returns only paths that exist, normalized