Make the console speak the backend's telemetry protocol

The telemetry store and the printer card described a protocol no backend
ever spoke. The store expected `{ type, data: { printerId, progressPct,
state } }` with a closed five-value state enum; `/ws/telemetry` sends
`{ type: "printer.telemetry", printer: {...} }` with `id`,
`progressPercent` and an open `status`, and prefaces it with a `ready`
frame. The card also displayed `printer.model`, which the printer
resource has never had - it has `vendor`.

Authentication was the other half: the socket now connects with the
subprotocol list `["bearer", token]`, which is what the backend gates
the upgrade on, and never a query parameter. The token is read from the
session on each attempt, because supabase-js refreshes it hourly and a
reconnect replaying the first token would work right up until it
mattered. With no session it does not connect at all, and app.vue drives
connect/disconnect from the session rather than a one-shot onMounted.

Reconnection is now backed off from 1s to 30s instead of a flat 3s. A
browser cannot see why an upgrade failed - a rejected token and an
unreachable server are both just onerror then onclose - so a token the
server will never accept previously retried every 3s for as long as the
tab stayed open.

`disconnect()` no longer reconnects the socket it just closed, via a
`wanted` flag; handlers from a replaced socket are ignored.

Null is no longer rendered as zero. A claimed printer whose gateway has
never reported has no temperature, and 0 degrees is a different claim:
null readings stay out of the graph's history, the labels show a dash,
the progress bar is omitted rather than drawn empty, and a null status
reads "unknown" rather than the old "offline", which asserted something
nobody had observed. `lastSeenAt` is shown, since it is what separates
an idle printer from one nothing has heard from in an hour.

The card takes one `printer` prop instead of a row plus a `live`
overlay - the push and the REST row are the same shape by design, so
pages/index.vue replaces one with the other and @continuum/ui stays
plain Vue that knows nothing of stores or the API. Status styling now
covers all six values of the backend's printer_status enum; paused and
maintenance previously drew as ordinary text.

Types are hand-mirrored in packages/ui/src/types.ts from
continuum-schemas, the convention this project already uses across
TS and Rust. @continuum/ts-types is in another repository and is not
resolvable here; a `link:` out of the repo would break on any checkout
not laid out like this machine's. Field names are verified identical to
the schema's.

Typechecks clean (nuxt typecheck, vue-tsc). Not exercised at runtime;
the protocol it now implements is the one verified live by
continuum-backend's scripts/e2e-edge.sh.

apps/web/app.vue

@@ -3,9 +3,17 @@ import { useTelemetryStore } from "~/stores/telemetry";
 
 const telemetry = useTelemetryStore();
 const user = useSupabaseUser();
+const session = useSupabaseSession();
 
+// Driven by the session rather than a one-shot onMounted: the session can
+// arrive after mount, and sign-out should drop the socket instead of leaving
+// it running on a token that is about to stop being valid.
 onMounted(() => {
-  telemetry.connect();
+  watch(
+    () => session.value?.access_token,
+    (token) => (token ? telemetry.connect() : telemetry.disconnect()),
+    { immediate: true },
+  );
 });
 
 onBeforeUnmount(() => {
@@ -18,7 +26,13 @@ onBeforeUnmount(() => {
     <header class="app-shell__header">
       <span class="app-shell__brand">Continuum</span>
       <span class="app-shell__status" :class="telemetry.connected ? 'is-online' : 'is-offline'">
-        {{ telemetry.connected ? "Live" : "Reconnecting…" }}
+        {{
+          telemetry.link === "connected"
+            ? "Live"
+            : telemetry.link === "connecting"
+              ? "Connecting…"
+              : "Offline"
+        }}
       </span>
       <span v-if="user" class="app-shell__user">{{ user.email }}</span>
     </header>