fix(ci): remove stale .importlinter shadowing real contracts, bump biome schema

- .importlinter (from the certamen-framework merge) referenced modules
  that no longer exist (certamen.config, certamen.models,
  certamen.certamen). import-linter prefers this standalone file over
  pyproject.toml's [tool.importlinter] section, so it crashed on every
  run and the Architecture & Import Contracts CI job silently never
  enforced the correct, current layer-boundary contracts already
  defined in pyproject.toml. Removing it lets those 4 contracts run for
  real (verified: 4 kept, 0 broken).
- frontend/biome.json $schema still pointed at 2.5.0 after the biome
  2.5.1 dependency bump (#68), tripping a CI schema-mismatch warning.

Author: nikolay-e

.importlinter

@@ -1,5 +1,5 @@
 {
-  "$schema": "https://biomejs.dev/schemas/2.5.0/schema.json",
+  "$schema": "https://biomejs.dev/schemas/2.5.1/schema.json",
   "vcs": { "enabled": true, "clientKind": "git", "useIgnoreFile": true },
   "files": { "includes": ["**", "!dist", "!src/generated"] },
   "formatter": {