feat: client and server logic to manage announcements (#3)

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### Attestations

- [x] I have read and agree to the [Code of Conduct](https://docs.nhcarrigan.com/community/coc/)
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- [x] I have pinned the dependencies to a specific patch version.

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- [x] I have run the linter and resolved any errors.
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### Tests

- [ ] My contribution adds new code, and I have added tests to cover it.
- [ ] My contribution modifies existing code, and I have updated the tests to reflect these changes.
- [ ] All new and existing tests pass locally with my changes.
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Reviewed-on: #3
Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Co-committed-by: Naomi Carrigan <commits@nhcarrigan.com>

client/src/app/announcements.ts

@@ -0,0 +1,38 @@
+/**
+ * @copyright nhcarrigan
+ * @license Naomi's Public License
+ * @author Naomi Carrigan
+ */
+import { Injectable } from "@angular/core";
+
+@Injectable({
+  providedIn: "root",
+})
+export class AnnouncementsService {
+  public constructor() {}
+  // eslint-disable-next-line @typescript-eslint/class-methods-use-this -- Getter for static URL.
+  private get url(): string {
+    return "http://localhost:20000/announcements";
+  }
+
+  public async getAnnouncements(): Promise<
+    Array<{
+      title:     string;
+      content:   string;
+      createdAt: string;
+      type:      "products" | "community";
+    }>
+  > {
+    const response = await fetch(this.url);
+    if (!response.ok) {
+      return [];
+    }
+
+    return (await response.json()) as Array<{
+      title:     string;
+      content:   string;
+      createdAt: string;
+      type:      "products" | "community";
+    }>;
+  }
+}