Implement Path Verification Utility in Platform Layer

[baync180705]

Migrate and enhance the path validation logic from system/utils to the new internal/platform package. This function is required to safely handle user inputs before backup operations.

Requirements:

• Create File: internal/platform/paths.go
• Functionality: Implement VerifyInputPath(rawPath string) (string, error) which must:
  -- Sanitize input strings (remove quotes/whitespace).
  -- Expand tilde (~) to the home directory.
  -- Resolve to an absolute path.
  -- Confirm the file exists on disk (return error if missing).

Usage Scope:

• WHERE TO USE: Strictly for Backup/Ingestion (e.g., validating custom key paths from CLI during ssh/gpg backup).
• WHERE NOT TO USE: Do not use for Restoration targets (files that don't exist yet).