[Feature] PDF Password Protect

### Summary
Add password encryption to PDF before download.

### Files to create/edit
- `src/pages/tools/PdfProtect.tsx` → create
- `src/App.tsx` → add import + route `/tools/pdf-protect`
- `src/pages/Tools.tsx` → add to `allTools[]`

### Implementation notes
- Use `pdf-lib` for encryption
- Input: PDF file + password + confirm password
- Show password strength indicator

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