rebrand to OffScale, and fix what the first extraction missed

Offscale was the first plugin extracted and it was done before we knew what
"extracted" meant. Music, done last, is the standard. This brings offscale to it.

── The rebrand ──

The plugin was `offscale` to the platform and `headscale` to itself: sidecar
name and handles, the port announcement, the API proxy name, the React
components, every hook, the react-query keys, the panel ids and appTypes, and
the Postgres table. Now all of those say offscale.

The line drawn, and it is deliberate: OffScale is Officer's tooling layer, and
Headscale is the server it manages. So every IDENTIFIER is offscale, while a
message like `headscale unreachable`, the `headscale apikeys create` hint and the
ACL assistant's prompt still say Headscale — because they are talking about the
remote server, and renaming them would make the code lie about what it reached.
495 occurrences became 180, and the 180 are all of that second kind.

── The live bug this uncovered ──

`headscaleSectionPath` built links to `/headscale/<section>`. The shell has no
such route — plugin routes come from `plugin.route`, which is `/offscale` — and
it redirects unknown paths to the home page. So every section link in the nav,
the console and the server picker silently went home. The extraction moved the
route and left the link builder behind.

Also live: ServersView told the user to run
`pm2 start ecosystem.config.cjs --only officer-headscale`, a process that has not
existed since the sidecar was renamed.

── The correctness fix music already had ──

api/router.ts hardcoded `prefix: '/api/offscale'`. The proxy strips
`prefix.length` characters, so a literal is correct only for a first-party
publisher; published by anyone else this mounts at `/api/p/<publisher>/offscale`
and forwards the wrong subpath. Derived from `mountPrefix()` now, as music does.

── The rest ──

- assets/icon.png — the OffScale artwork, 256px to match music's. The tile stops
  being a glyph badge.
- First tests: 21 of them, over the version floor and the protobuf normalisers.
  Those are the two places a Headscale release actually breaks this, and they had
  no coverage at all. `meetsFloor` has a real trap pinned now — comparing minor
  first would refuse 1.0 as older than 0.29.
- OFFSCALE_API.md — the contract was a 45-line comment inside sidecar/index.ts,
  which is not linkable and not published. Now a document, as MUSIC_API.md is.
- web/panels.ts re-exported three components. A plugin cannot export components;
  that was residue of the platform importing them before extraction.
- Comments pointed at src/servers/api/headscale/ and src/servers/sidecar/headscale/,
  neither of which has existed since the extraction.

The crypto purpose moved headscale → offscale too, and the secret-store row was
renamed rather than left to create a fresh key — the material is preserved, so
this is reversible. Free to do only because offscale_servers had 0 rows; with one
stored API key it would have been a migration.

sidecar/companion.ts

@@ -1,9 +1,9 @@
-import { getActiveHeadscaleCredentials, type HeadscaleServerCredentials } from '../db/queries';
+import { getActiveOffscaleCredentials, type OffscaleServerCredentials } from '../db/queries';
 import { badRequest, methodNotAllowed, notFound, type OfficerContext } from './routes';
 
 // The Officer Companion API — a small service deployed NEXT TO each Headscale server that answers what the
 // admin API structurally cannot: is the container up, what do its logs say, and start/stop/restart it.
-// Contract: COMMS/HEADSCALE_COMPANION_API.md.
+// Contract: COMMS/OFFSCALE_COMPANION_API.md.
 //
 // Three facts shape everything here.
 //
@@ -38,7 +38,7 @@ type CompanionCall = { path: string; method?: string; body?: unknown; timeoutMs?
  * "unreachable" is information rather than a failure.
  */
 export async function callCompanion(
-  creds: HeadscaleServerCredentials,
+  creds: OffscaleServerCredentials,
   { path, method = 'GET', body, timeoutMs = DEFAULT_TIMEOUT_MS, signal }: CompanionCall,
 ): Promise<Response | string> {
   let res: Response;
@@ -85,8 +85,8 @@ export async function readBody(res: Response): Promise<Record<string, unknown> |
 }
 
 /** The active server's credentials, or a 409 the UI already knows how to render. */
-export async function activeCreds(userId: number): Promise<HeadscaleServerCredentials | Response> {
-  const creds = await getActiveHeadscaleCredentials(userId);
+export async function activeCreds(userId: number): Promise<OffscaleServerCredentials | Response> {
+  const creds = await getActiveOffscaleCredentials(userId);
   if (!creds) {
     return Response.json({ error: 'no active Headscale server', code: 'no_active_server' }, { status: 409 });
   }
@@ -94,7 +94,7 @@ export async function activeCreds(userId: number): Promise<HeadscaleServerCreden
 }
 
 /** `GET /_officer/companion/health` — verdict, container state and, when unhappy, its own diagnosis. */
-async function health(creds: HeadscaleServerCredentials): Promise<Response> {
+async function health(creds: OffscaleServerCredentials): Promise<Response> {
   const res = await callCompanion(creds, { path: '/health' });
   if (typeof res === 'string') return Response.json(unavailable(res));
 
@@ -106,7 +106,7 @@ async function health(creds: HeadscaleServerCredentials): Promise<Response> {
 }
 
 /** `GET /_officer/companion/logs?tail=N` — a snapshot of the last N lines. */
-async function logs(creds: HeadscaleServerCredentials, url: URL): Promise<Response> {
+async function logs(creds: OffscaleServerCredentials, url: URL): Promise<Response> {
   const tail = Number(url.searchParams.get('tail') ?? 200);
   if (!Number.isInteger(tail) || tail < 1 || tail > 2000) return badRequest('tail must be 1–2000');
 
@@ -127,7 +127,7 @@ async function logs(creds: HeadscaleServerCredentials, url: URL): Promise<Respon
  * ReadableStream that Bun cancels when the client disconnects, which aborts the upstream fetch in turn.
  * Buffering it into frames here would break that, and would also mean a log line waiting on our own flush.
  */
-async function logStream(creds: HeadscaleServerCredentials, ctx: OfficerContext): Promise<Response> {
+async function logStream(creds: OffscaleServerCredentials, ctx: OfficerContext): Promise<Response> {
   const tail = Number(ctx.url.searchParams.get('tail') ?? 200);
   if (!Number.isInteger(tail) || tail < 1 || tail > 2000) return badRequest('tail must be 1–2000');
 
@@ -165,7 +165,7 @@ const ACTIONS = new Set(['restart', 'stop', 'start']);
  * Every one of these drops every node's control-plane connection for the duration. That is the intended
  * "kill it" behaviour and the reason the UI asks twice; it is not something to retry automatically.
  */
-async function action(creds: HeadscaleServerCredentials, name: string): Promise<Response> {
+async function action(creds: OffscaleServerCredentials, name: string): Promise<Response> {
   // 60s: docker restart on a busy container is not fast, and a timeout here reads as "did it work?" — the
   // one question this feature exists to answer.
   const res = await callCompanion(creds, { path: `/${name}`, method: 'POST', timeoutMs: 60_000 });