Fix team name blink on navigation by lifting teams into a singleton store

Teams list was fetched on every Sidebar mount (each page navigation),
causing a flash from '…' to the real name on every tab switch. Move teams
into a module-level reactive store (teams.svelte.ts) that fetches once per
session and is shared between Sidebar and the team page.

frontend/src/lib/teams.svelte.ts

@@ -0,0 +1,31 @@
+import { listTeams, type TeamWithRole } from '$lib/api/team';
+
+function createTeamsStore() {
+	let teams = $state<TeamWithRole[]>([]);
+	let loaded = $state(false);
+
+	return {
+		get list() {
+			return teams;
+		},
+		get loaded() {
+			return loaded;
+		},
+		async fetch() {
+			if (loaded) return;
+			const result = await listTeams();
+			if (result.ok) {
+				teams = result.data;
+				loaded = true;
+			}
+		},
+		// Call after mutating teams (create/switch triggers a full reload, but
+		// adding a team locally avoids a flicker in the popover list).
+		set(newTeams: TeamWithRole[]) {
+			teams = newTeams;
+			loaded = true;
+		}
+	};
+}
+
+export const teams = createTeamsStore();