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>

src/template.ts

@@ -22,6 +22,8 @@ export type RenderMode =
   | 'half_vertical'
   | 'quadrant';
 export interface Renderable {
+  hash: string;
+  nextEvent?: Date;
   render(mode: RenderMode, model: DeviceModel): Promise<string>;
 }