fix: replace per-path proxy with cookie-based catch-all routing

The /session/{id} URL prefix collided with OpenCode's internal
/session/{slug} SPA routes, causing a blank page. Now /c/{id} is
a thin entry point that sets a session cookie and redirects to /,
where the SPA loads at root with its router working correctly.

This also replaces ~50 individual per-path proxy route handlers
with a single /{path:path} catch-all, and simplifies the Caddyfile
from ~180 lines to ~17.

Makefile

@@ -52,7 +52,7 @@ try: up
 		sleep 1; \
 	done && \
 	echo "" && \
-	echo "Opening http://localhost:8080/session/$$SESSION_ID" && \
-	(xdg-open "http://localhost:8080/session/$$SESSION_ID" 2>/dev/null || \
-	open "http://localhost:8080/session/$$SESSION_ID" 2>/dev/null || \
-	echo "Visit: http://localhost:8080/session/$$SESSION_ID")
+	echo "Opening http://localhost:8080/c/$$SESSION_ID" && \
+	(xdg-open "http://localhost:8080/c/$$SESSION_ID" 2>/dev/null || \
+	open "http://localhost:8080/c/$$SESSION_ID" 2>/dev/null || \
+	echo "Visit: http://localhost:8080/c/$$SESSION_ID")