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>

CMakeLists.txt

@@ -295,6 +295,29 @@ else()
     set(CURL_INCLUDE_DIRS ${CURL_INCLUDE_DIR})
 endif()
 
+# libwebp - WebP decode (still + animated via WebPAnimDecoder). Built from source, static, decode-only
+# so Linux / mingw-Windows / macOS-osxcross all build it identically (the mingw/osx sysroots have no
+# webp). Encode tools are disabled to avoid pulling in libpng/zlib that the cross sysroots lack.
+message(STATUS "Fetching libwebp (decode-only, static)...")
+FetchContent_Declare(
+    libwebp
+    GIT_REPOSITORY https://github.com/webmproject/libwebp.git
+    GIT_TAG v1.4.0
+    GIT_SHALLOW TRUE
+)
+set(WEBP_LINK_STATIC ON CACHE BOOL "" FORCE)
+set(WEBP_BUILD_ANIM_UTILS OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_CWEBP OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_DWEBP OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_GIF2WEBP OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_IMG2WEBP OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_VWEBP OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_WEBPINFO OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_LIBWEBPMUX OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_WEBPMUX OFF CACHE BOOL "" FORCE)
+set(WEBP_BUILD_EXTRAS OFF CACHE BOOL "" FORCE)
+FetchContent_MakeAvailable(libwebp)
+
 # libsodium - platform-specific
 # Search order per platform:
 #   1. Local pre-built in libs/libsodium{-mac,-win}/  (downloaded by scripts/fetch-libsodium.sh)
@@ -735,6 +758,7 @@ target_include_directories(ObsidianDragon PRIVATE
     ${GLAD_INCLUDE}
     ${CURL_INCLUDE_DIRS}
     ${MINIZ_DIR}
+    ${libwebp_SOURCE_DIR}/src   # <webp/decode.h>, <webp/demux.h> (FetchContent build tree)
 )
 
 target_link_libraries(ObsidianDragon PRIVATE
@@ -744,6 +768,8 @@ target_link_libraries(ObsidianDragon PRIVATE
     sqlite3_amalgamation
     ${CURL_LIBRARIES}
     ${SODIUM_LIBRARY}
+    webp
+    webpdemux   # WebPAnimDecoder (animated WebP); transitively pulls in webp + sharpyuv
 )
 
 if(DRAGONX_LITE_BACKEND_READY)