feat: actional proposals segment store (dfinity#4608)

# Motivation

Adds a new store to contain current selection state of
"all"|"actionable" segment controller. Should be "all" be default, same
for all projects and not preserved in the local storage. Will be used
here dfinity#4591

# Changes

- `actionableProposalsSegmentStore`

# Tests

- `actionableProposalsSegmentStore`

# Todos

- [ ] Add entry to changelog (if necessary).
Not yet.

frontend/src/lib/stores/actionable-proposals-segment.store.ts

@@ -0,0 +1,39 @@
+import { writable, type Readable } from "svelte/store";
+
+export type ActionableSegmentSelection = "all" | "actionable";
+
+export interface ActionableProposalsSegmentStoreData {
+  selected: ActionableSegmentSelection;
+}
+
+export interface ActionableProposalsSegmentStore
+  extends Readable<ActionableProposalsSegmentStoreData> {
+  set: (selected: ActionableSegmentSelection) => void;
+  resetForTesting: () => void;
+}
+
+/**
+ * A store that contains selected state of actionable proposals segment.
+ * By default, it's "all" and not stored in LocalStorage.
+ */
+const initActionableProposalsSegmentStore =
+  (): ActionableProposalsSegmentStore => {
+    const { subscribe, set } = writable<ActionableProposalsSegmentStoreData>({
+      selected: "all",
+    });
+
+    return {
+      subscribe,
+
+      set(selected: ActionableSegmentSelection) {
+        set({ selected });
+      },
+
+      resetForTesting(): void {
+        set({ selected: "all" });
+      },
+    };
+  };
+
+export const actionableProposalsSegmentStore =
+  initActionableProposalsSegmentStore();

frontend/src/tests/lib/stores/actionable-proposals-segment.store.spec.ts

@@ -0,0 +1,20 @@
+import { actionableProposalsSegmentStore } from "$lib/stores/actionable-proposals-segment.store";
+import { get } from "svelte/store";
+
+describe("actionable proposals segment store", () => {
+  beforeEach(() => {
+    actionableProposalsSegmentStore.resetForTesting();
+  });
+
+  it('should have "all" by default ', () => {
+    expect(get(actionableProposalsSegmentStore).selected).toEqual("all");
+  });
+
+  it("should set selected", () => {
+    actionableProposalsSegmentStore.set("actionable");
+    expect(get(actionableProposalsSegmentStore).selected).toEqual("actionable");
+
+    actionableProposalsSegmentStore.set("all");
+    expect(get(actionableProposalsSegmentStore).selected).toEqual("all");
+  });
+});