fix: move dependencies back under [project] in pyproject.toml

The key had drifted below [project.urls], making setuptools interpret it
as a URL entry and failing validation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -8,6 +8,9 @@ version = "5.3.4"
 description = "Heartbeat monitoring system — client (hbc) and server (hbd)"
 readme = "README.md"
 requires-python = ">=3.11"
+dependencies = [
+  "PyYAML>=6.0",
+]
 license = "MIT"
 license-files = ["LICENSE.md"]
 keywords = ["heartbeat", "monitoring", "dns", "websocket", "system-monitoring"]
@@ -29,11 +32,6 @@ classifiers = [
 [project.urls]
 Repository = "https://git.wrede.ca/andreas/heartbeat"
 
-# Core dependencies (required for both client and server)
-dependencies = [
-  "PyYAML>=6.0",
-]
-
 [project.optional-dependencies]
 # Client-only dependencies (hbc - system monitoring client)
 client = [