fix: add cross-platform path validation and graceful error handling

[samalbishnupriya06-stack]

Summary

Addresses the input validation and cross-platform path handling issue.
The tool previously risked crashes on malformed paths or mismatched OS separators.

Changes Made

Backend (src-tauri/src/main.rs)

• Added validate_path() helper using std::path::Path for OS-agnostic path resolution
• Added path traversal protection (../ stripping)
• Wrapped all fs:: operations with match e.kind() to return human-readable errors instead of panics
• Added explicit checks for: empty input, non-existent paths, permission errors, not-a-directory/file

Frontend (src/utils/pathValidator.ts) — New file

• Client-side validation layer before any invoke() call
• Detects Windows vs Unix paths via heuristics
• Rejects: empty strings, null bytes, Windows invalid chars, path traversal

Frontend (src/services/optimizer.ts)

• All Tauri invocations now validate + normalize paths first
• Errors surfaced from Rust are caught and re-thrown as readable Error objects

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature
• Improvement (code quality / stability)

How to Test

1. Run npm run tauri dev
2. Enter an invalid path like ../../etc/passwd → should show validation error, not crash
3. Enter a non-existent path → should show "Path does not exist" message
4. Test on both Windows and macOS/Linux — path separators should be handled correctly

Checklist

• Code follows the project's TypeScript + Rust style guides
• cargo check passes with no errors
• npm run type-check passes
• Tests added (can be added in follow-up if test suite is set up)
• No breaking changes to existing API surface

Related Issue

Closes #34