feat(images): WebP decode via libwebp + more stb formats

Broaden avatar/image support:

- Add libwebp (FetchContent, static, decode-only) so WebP loads. Built from
  source for Linux / mingw-Windows / macOS-osxcross identically — the cross
  sysroots have no webp, so vendoring from source is the one portable path;
  encode/tool builds are disabled to avoid pulling in libpng/zlib. Linked as
  webp + webpdemux (the latter for animated WebP, wired next).
- texture_loader routes all decode through DecodeImageRGBA: sniffs the RIFF/
  WEBP header and uses libwebp (WebPDecodeRGBAInto into a free()-able buffer),
  else stb — so every existing caller (avatars, QR, thumbnails) gains WebP for
  free with no allocator mismatch.
- Enable stb's TGA / PSD / PNM / PIC decoders and add .webp/.tga/.psd/.pnm/
  .ppm/.pgm/.pic to the image-picker + avatar-library extension lists.

Verified: libwebp builds static and a real .webp decodes to correct RGBA.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/windows/image_picker.h

@@ -297,7 +297,8 @@ private:
     static bool isImageName(const std::string& name) {
         auto lower = name;
         for (char& c : lower) c = (char)std::tolower((unsigned char)c);
-        static const char* kExt[] = { ".png", ".jpg", ".jpeg", ".bmp", ".gif" };
+        static const char* kExt[] = { ".png", ".jpg", ".jpeg", ".bmp", ".gif", ".webp",
+                                      ".tga", ".psd", ".pnm", ".ppm", ".pgm", ".pic" };
         for (const char* e : kExt) {
             const std::string ext(e);
             if (lower.size() > ext.size() && lower.compare(lower.size()-ext.size(), ext.size(), ext) == 0)