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/format.js

@@ -98,6 +98,14 @@ export function severityClass(sev) {
   return '';
 }
 
+// A stable identity colour for a team, so the same team always reads the same
+// colour without the server needing to store one. Teams have no colour field;
+// this hashes the id into the six-colour rcN palette app.css already has for
+// the rota's per-person chips (a team is not a status, so never severity).
+export function teamColorClass(teamID) {
+  return `rc${(((teamID % 6) + 6) % 6) + 1}`;
+}
+
 // A one-line summary of the group labels, without the one the title already shows.
 export function labelSummary(labels, skip = 'alertname') {
   return Object.entries(labels || {})