fix: resolve 404 on tool pages and stabilize Tauri CI

- Add missing tool-content entries for djvu-to-pdf, fb2-to-pdf, markdown-to-pdf and xps-to-pdf in en.ts so getToolContent no longer returns undefined and triggers notFound().

- Harden build-tauri workflow against intermittent crates.io schannel failures on Windows runners: enable sparse registry protocol, increase CARGO_NET_RETRY, disable HTTP multiplexing, use git CLI for fetch, add Swatinem/rust-cache, and wrap the build step with nick-fields/retry.

fix(upload): resolve file drag-and-drop issues on Linux Chrome (Wayla…

…nd/GNOME) and Tauri desktop environment

Update LibreOffice converter implementation

Merge pull request PDFCraftTool#30 from eq1024/main

fix(pdf): add automatic fallback mechanism for failed image embedding