Fix PDF previews for Windows extended-length paths

[jsgrrchg]

Windows vault paths canonicalized by Rust use the extended-length prefix \\?\. The preview URL builder treated its ? as the start of a URL query, truncated the file path, and rejected PDFs inside the vault as being outside it before PDF.js could load them.

Preserve the Windows namespace when splitting URL suffixes, including extended UNC paths and forward-slash equivalents. Add regression coverage for PDF loading, preview authorization, generated image URLs, and files outside the vault.

Include a standalone integration suite that compiles a small Rust canonicalization helper, reads a generated PDF through the real preview handler, and loads its page operators with PDF.js. Run it with npx vitest run --config scripts/pdf-windows-paths.vitest.config.ts from apps/desktop with Rust installed. The OneDrive-like test directory is local; it does not exercise cloud synchronization.

Validation:

• PDF path unit and integration checks passed on both Windows and Linux in GitHub Actions. The temporary workflow has been removed after validation; the tests remain available.
• 59 focused preview/viewer tests and 2 filesystem/PDF.js integration tests passed locally on Linux.
• Five new regression cases fail with the original implementation and pass with the fix.
• TypeScript build checks, ESLint for changed TypeScript files, and diff whitespace checks passed.
• The user's exact environment has not been reproduced.

Related to #519.