thumbnail preview fixes windows (#6074)

frontend/src-tauri/thumbnail-handler/README.md

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+# Windows PDF Thumbnail Handler
+
+A lightweight COM DLL that provides PDF page-preview thumbnails in Windows Explorer when Stirling-PDF is the default PDF application.
+
+## Why this exists
+
+When Stirling-PDF registers as the default PDF handler, Windows associates `.pdf` files with Stirling's ProgID. Without a thumbnail handler on that ProgID, Explorer falls back to showing the application icon (the big S logo) instead of a page preview. This DLL restores thumbnail previews by implementing the Windows Shell `IThumbnailProvider` COM interface.
+
+## How it works
+
+1. **Explorer requests a thumbnail** — when a folder with PDFs is opened in Medium/Large icon view, Explorer loads the DLL via the registered COM CLSID.
+2. **Shell calls `IInitializeWithStream`** — passes the PDF file content as an `IStream`.
+3. **Shell calls `IThumbnailProvider::GetThumbnail(cx)`** — requests a bitmap of size `cx × cx`.
+4. **The DLL renders page 1** using the built-in `Windows.Data.Pdf` WinRT API (the same engine Edge uses), preserving aspect ratio.
+5. **WIC decodes the rendered PNG** into BGRA pixels, which are copied into an `HBITMAP` via `CreateDIBSection`.
+6. **Explorer displays the bitmap** as the file's thumbnail.
+
+All COM methods are wrapped in `catch_unwind` so a malformed PDF cannot crash Explorer.
+
+## Technical details
+
+| | |
+|---|---|
+| **Language** | Rust (cdylib) |
+| **DLL size** | ~156 KB |
+| **External deps** | None — uses only Windows built-in APIs |
+| **PDF renderer** | `Windows.Data.Pdf` (WinRT, Windows 10+) |
+| **Image decode** | WIC (`IWICImagingFactory`) with BGRA32 format conversion |
+| **COM CLSID** | `{2D2FBE3A-9A88-4308-A52E-7EF63CA7CF48}` |
+| **Threading model** | Apartment (STA — standard for shell extensions) |
+| **Min Windows** | Windows 10 |
+
+## Registry entries (managed by MSI)
+
+The WiX installer (`provisioning.wxs`) registers:
+
+- **CLSID** at `HKLM\SOFTWARE\Classes\CLSID\{2D2FBE3A-...}\InprocServer32` pointing to the DLL
+- **Shellex** at `HKLM\SOFTWARE\Classes\.pdf\shellex\{E357FCCD-...}` linking `.pdf` thumbnails to our CLSID
+
+Both are automatically removed on uninstall.
+
+## Building
+
+The DLL is built automatically as part of the Tauri build pipeline via `build-provisioner.mjs`:
+
+```bash
+cd frontend
+npm run tauri-build
+```
+
+To build the DLL standalone:
+
+```bash
+cd frontend/src-tauri/thumbnail-handler
+cargo build --release
+# Output: target/release/stirling_thumbnail_handler.dll
+```
+
+## Linux / macOS
+
+This DLL is Windows-only. Linux and macOS don't need it — their thumbnail systems (thumbnailers on Linux, Quick Look on macOS) are decoupled from the default app association and continue working regardless of which app is set as default.