Fix expired token redirect loop and theme error pages (v0.6.10)

Add JWT expiry check in OAuth PKCE middleware to prevent infinite redirect
loops when the landing page (exempt from _AuthMiddleware) has an expired
token cookie. Extract shared HTML styles to _common.py and apply branded
theme to OAuth error page.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rustyconover

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "vgi-rpc"
-version = "0.6.9"
+version = "0.6.10"
 description = "Vector Gateway Interface - RPC framework based on Apache Arrow"
 readme = "README.md"
 requires-python = ">=3.13"