[fix](trx-wefax): populate WEFAX tab with working live canvas and gallery

- Fix WefaxProgress.line_data serialization: change from Vec<u8> (JSON
  array) to base64-encoded String so the browser's atob() call works
- Set output_dir in server WefaxConfig to $XDG_CACHE_HOME/trx-rs/wefax
  so decoded PNG images are persisted to disk
- Add /images/{filename} GET route in trx-frontend-http to serve saved
  WEFAX PNGs with path traversal protection
- Capture live canvas as data URI on image completion for immediate
  gallery thumbnail display without requiring the file serving route

https://claude.ai/code/session_01V1kLpgLPb8Q5wSv4UrcLbr
Signed-off-by: Claude <noreply@anthropic.com>

Cargo.lock

@@ -3265,6 +3265,7 @@ dependencies = [
 name = "trx-wefax"
 version = "0.1.0"
 dependencies = [
+ "base64",
  "png",
  "trx-core",
 ]