Add configurable plugin system and move hardcoded IP to config

Introduces a plugin registry so the display mode is selectable per
device via the NixOS module config (defaults to "calendar"). Moves
the hardcoded render URL base into config.base_url. Adds tests
exercising the plugin system with a synthetic ICS feed.

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

default.nix

@@ -35,10 +35,16 @@ in
       type = json.type;
       example = lib.literalExpression ''
         {
-          unknown = {
-            urls = ["https://user.fm/calendar/....ics"];
-            refresh = 60;
-            model = "inkplate_10";
+          base_url = "http://192.168.50.124:2300";
+          devices = {
+            unknown = {
+              plugin = "calendar";
+              settings = {
+                urls = ["https://user.fm/calendar/....ics"];
+              };
+              refresh = 60;
+              model = "inkplate_10";
+            };
           };
         }
       '';