Embed webapp into the binary with fs-embed, drop --static-dir

ottyl was requiring webapp/dist to be shipped alongside the binary and
served via ServeDir/ServeFile pointed at a --static-dir CLI arg. Embed
the built webapp at compile time instead, so ottyl is a single
self-contained executable with no runtime static directory dependency.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

Cargo.toml

@@ -17,6 +17,7 @@ chrono = "0.4"
 humantime = "2"
 directories = "6.0.0"
 fs-embed = "0.2.4"
+include_dir = "0.7.4"
 
 [dev-dependencies]
 tempfile = "3.27.0"