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/index.html

@@ -86,6 +86,9 @@
         <img src="/icon.svg" alt="" width="28" height="28">
         <span>terdut</span>
       </a>
+      <!-- Which team the app is scoped to. Hidden unless the signed-in user is
+           in more than one; teamselector.js fills it in and wires the click. -->
+      <button class="nav-team-selector" id="team-selector" type="button" hidden></button>
       <a class="nav-link" href="/" data-section="queue" aria-label="Queue">
         <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M4 6h16M4 12h16M4 18h10"/></svg>
         <span class="nav-label">Queue</span>
@@ -126,6 +129,7 @@
           <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M4 7h16M4 12h16M4 17h16"/></svg>
           <span class="nav-badge menu-btn-badge" data-badge hidden></span>
         </button>
+        <button class="team-selector-mobile" id="team-selector-mobile" type="button" hidden></button>
         <h1 class="topbar-title" id="topbar-title">Queue</h1>
       </div>
       <span class="open-pill" id="open-pill" hidden></span>