finish phase 4: drop @atproto/api dependency, lex-only errors, delete toLex and legacy validators

oxlint-suppressions.json gains a baseline entry for webpack.config.js: the
file was never previously staged/linted (lint script covers src+modules
only), and editing it exposed its pre-existing violations to lint-staged.
All suppressed counts are pre-existing; this change only removes a line.

package.json

@@ -98,7 +98,6 @@
     "prettier": "prettier --check ."
   },
   "dependencies": {
-    "@atproto/api": "0.20.34",
     "@atproto/common-web": "0.5.6",
     "@atproto/lex": "0.2.2",
     "@atproto/lex-client": "0.2.4",