[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>

src/decoders/trx-wefax/Cargo.toml

@@ -9,4 +9,5 @@ edition = "2021"
 
 [dependencies]
 trx-core = { path = "../../trx-core" }
+base64 = "0.22"
 png = "0.17"