Add a global, colour-coded team selector to the nav

state.js's currentTeam() was hard-coded to teams[0] and never really meant
"the team currently selected" — team.js's settings page and queue.js's
filter chips each kept their own separate, unsynchronized notion of "which
team" instead, so picking one on one page had no effect on the other.

Replaces both with a single state.selectedTeamID, set only through the new
setSelectedTeam (persisted in localStorage, unlike the queue's old per-tab
sessionStorage filter) and broadcast to listeners via onTeamChange. A new
teamselector.js control — a coloured dot plus the team's name, or "All
teams" — sits at the top of both the desktop sidebar and the mobile topbar,
opening the existing bottom-sheet menu to switch. Shown only once someone is
in more than one team, matching every other team-aware control in this app.

Colours come from a new teamColorClass() in format.js, hashing a team's id
into the six-colour rc1..rc6 palette already used for the rota's per-person
chips, so no schema or API change is needed. The queue's team filter chips
pick up the same colours.

internal/web/static/js/state.js

@@ -10,12 +10,53 @@ export const state = {
   auth: { password_login: true, oidc: { enabled: false, name: '' } },
   open: [], // the default queue: open, not snoozed
   teams: [], // the teams the viewer belongs to, each with their role
+  // Which team the whole app is scoped to right now; null means "All teams".
+  // Set only through setSelectedTeam below, never assigned directly, so every
+  // view stays in sync and the choice is remembered across reloads.
+  selectedTeamID: loadSelectedTeam(),
 };
 
-// The team whose schedule and settings the views act on. A viewer in one team —
-// which is everybody until somebody makes a second — never has to choose.
+const SELECTED_TEAM_KEY = 'terdut.selectedTeam';
+
+function loadSelectedTeam() {
+  try {
+    const raw = localStorage.getItem(SELECTED_TEAM_KEY);
+    return raw ? Number(raw) : null;
+  } catch {
+    return null; // storage unavailable, or nothing saved yet
+  }
+}
+
+// Callbacks to run whenever the selected team changes, so every view that
+// cares — the queue's filter, the Team settings page, the selector's own
+// trigger — stays in sync without a general event bus, following the one
+// precedent for this in the codebase: onboarding.js's onRerender.
+const teamListeners = [];
+export function onTeamChange(cb) {
+  teamListeners.push(cb);
+}
+
+// setSelectedTeam changes which team the app is scoped to (id, or null for
+// "All teams"), persists it — a durable preference, unlike the per-tab
+// sessionStorage filter this replaces — and tells every registered listener.
+export function setSelectedTeam(id) {
+  state.selectedTeamID = id;
+  try {
+    if (id == null) localStorage.removeItem(SELECTED_TEAM_KEY);
+    else localStorage.setItem(SELECTED_TEAM_KEY, String(id));
+  } catch {
+    /* storage unavailable */
+  }
+  for (const cb of teamListeners) cb();
+}
+
+// The team whose schedule and settings the views act on: the selected team,
+// falling back to the first one the viewer belongs to — which is everybody's
+// only team until somebody makes a second, or the stored selection naming a
+// team the account has since left.
 export function currentTeam() {
-  return state.teams[0] || null;
+  const teams = state.teams || [];
+  return teams.find((t) => t.id === state.selectedTeamID) || teams[0] || null;
 }
 
 export function myID() {
@@ -36,6 +77,12 @@ export async function users() {
 
 export async function loadTeams() {
   state.teams = await api.teams();
+  // A stored id that no longer names one of the account's teams — left it, or
+  // this is simply a different account signed in on the same browser — is as
+  // good as unset.
+  if (state.selectedTeamID != null && !state.teams.some((t) => t.id === state.selectedTeamID)) {
+    state.selectedTeamID = null;
+  }
   return state.teams;
 }