refactor: Standardize data processing and update to Svelte 5 runes

This commit introduces a major refactoring of the data processing logic across multiple modules (events, archives, posts, sponsorships) to use a standardized pattern with . This improves consistency and maintainability.

Key changes:
- Replaced module-specific data processing with a generic helper.
- Removed deprecated  functions.
- Updated Svelte components to leverage the new Svelte 5 runes, simplifying state management.
- Fixed linting errors and updated test configurations.
- Added .

package.json

@@ -84,6 +84,9 @@
     "vite": "^6.0.0",
     "vitest": "^3.0.0"
   },
+  "vitest": {
+    "exclude": ["tests"]
+  },
   "type": "module",
   "dependencies": {
     "@codemirror/commands": "^6.8.1",