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/app.js

@@ -11,6 +11,7 @@ import * as alerts from './alerts.js';
 import * as stats from './stats.js';
 import * as account from './account.js';
 import * as team from './team.js';
+import * as teamselector from './teamselector.js';
 import * as admin from './admin.js';
 import * as adminuser from './adminuser.js';
 import * as adminteam from './adminteam.js';
@@ -230,6 +231,7 @@ async function boot() {
   $('login-form').addEventListener('submit', onLogin);
   $('signup-form').addEventListener('submit', onSignup);
   $('menu-btn').addEventListener('click', openNavMenu);
+  teamselector.init();
   ssoErrorCode = takeSSOError();
 
   // /signup is the one route that works without a session.
@@ -245,6 +247,7 @@ async function boot() {
     await loadAuthConfig();
     state.me = await api.me();
     await loadTeams();
+    teamselector.render();
     // The Admin tab exists only for an administrator. Somebody who types /admin
     // anyway gets the view's own "ask an administrator" card, not a blank page.
     $('nav-admin').hidden = !state.me?.user?.is_admin;
@@ -328,6 +331,7 @@ async function onSignup(e) {
     history.replaceState({ depth: 0 }, '', '/');
     route = parseRoute('/');
     await loadTeams();
+    teamselector.render();
     $('nav-admin').hidden = !state.me?.user?.is_admin;
     showApp();
   } catch (ex) {