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.

db/schema.ts

@@ -2,7 +2,7 @@ import { pgTable, serial, integer, text, boolean, timestamp, uniqueIndex } from
 import { sql } from 'drizzle-orm';
 import { users } from 'officerdb/auth/schema';
 
-// The Headscale servers the owner manages, for the officer-headscale sidecar. Officer targets no single
+// The Headscale servers the owner manages, for the officer-offscale sidecar. Officer targets no single
 // Headscale: the owner registers one or more servers (URL + an admin API key generated on that server) and
 // toggles between them, so this is configuration the user creates at runtime rather than env vars.
 //
@@ -12,12 +12,12 @@ import { users } from 'officerdb/auth/schema';
 // at all. SECURITY_AUDIT.md L2 records plaintext credential storage as an open finding, so the plaintext
 // email/integrations tables are debt to avoid copying, not a precedent to follow.
 //
-// Every table here is `headscale_`-prefixed and this file holds nothing else: when sidecars own their own
-// schema it moves wholesale into src/servers/sidecar/headscale/ with no untangling. Only the
-// officer-headscale sidecar reads or writes these tables.
+// Every table here is `offscale_`-prefixed and this file holds nothing else: when sidecars own their own
+// schema it moves wholesale into ../sidecar/ with no untangling. Only the
+// officer-offscale sidecar reads or writes these tables.
 
-export const headscaleServers = pgTable(
-  'headscale_servers',
+export const offscaleServers = pgTable(
+  'offscale_servers',
   {
     id: serial('id').primaryKey(),
     userId: integer('user_id')
@@ -44,11 +44,11 @@ export const headscaleServers = pgTable(
   },
   (t) => [
     // One registration per URL — re-registering the same server should be an edit, not a duplicate.
-    uniqueIndex('uq_headscale_servers_user_url').on(t.userId, t.url),
+    uniqueIndex('uq_offscale_servers_user_url').on(t.userId, t.url),
     // At most one active server per owner, enforced by the DB rather than by convention: a partial unique
-    // index over the active rows only. setActiveHeadscaleServer still clears the others in a transaction,
+    // index over the active rows only. setActiveOffscaleServer still clears the others in a transaction,
     // but a bug there fails loudly here instead of silently leaving two servers active.
-    uniqueIndex('uq_headscale_servers_one_active')
+    uniqueIndex('uq_offscale_servers_one_active')
       .on(t.userId)
       .where(sql`${t.isActive}`),
   ],