refactor(subsonic): centralize the REST protocol version in one constant

The Subsonic protocol version (v=1.16.1) was hardcoded across six call sites.
Hoist it into a single SUBSONIC_API_VERSION constant next to SUBSONIC_CLIENT so
it can be reasoned about and adjusted in one place. No behaviour change.

src/api/subsonicClient.ts

@@ -8,6 +8,15 @@ import { findServerByIdOrIndexKey, resolveServerIdForIndexKey } from '../utils/s
 
 export const SUBSONIC_CLIENT = `psysonic/${version}`;
 
+/**
+ * Subsonic REST protocol version sent on every request. A server returns
+ * error 30 ("Incompatible REST protocol version") if the client asks for a
+ * HIGHER version than it supports — surfaced to the user via the connection
+ * error toast. OpenSubsonic extensions are negotiated via the `openSubsonic`
+ * response flag, not this number.
+ */
+export const SUBSONIC_API_VERSION = '1.16.1';
+
 export function secureRandomSalt(): string {
   const buf = new Uint8Array(8);
   crypto.getRandomValues(buf);
@@ -17,7 +26,7 @@ export function secureRandomSalt(): string {
 export function getAuthParams(username: string, password: string) {
   const salt = secureRandomSalt();
   const token = md5(password + salt);
-  return { u: username, t: token, s: salt, v: '1.16.1', c: SUBSONIC_CLIENT, f: 'json' };
+  return { u: username, t: token, s: salt, v: SUBSONIC_API_VERSION, c: SUBSONIC_CLIENT, f: 'json' };
 }
 
 export function restBaseFromUrl(serverUrl: string): string {

src/api/subsonicStreamUrl.ts

@@ -5,7 +5,7 @@ import type { CoverArtTier } from '../cover/types';
 import { useAuthStore } from '../store/authStore';
 import { connectBaseUrlForServer } from '../utils/server/serverEndpoint';
 import { findServerByIdOrIndexKey } from '../utils/server/serverLookup';
-import { restBaseFromUrl, SUBSONIC_CLIENT, secureRandomSalt } from './subsonicClient';
+import { restBaseFromUrl, SUBSONIC_CLIENT, SUBSONIC_API_VERSION, secureRandomSalt } from './subsonicClient';
 
 function coverArtQueryParams(username: string, password: string, id: string, size: number): URLSearchParams {
   const salt = secureRandomSalt();
@@ -16,7 +16,7 @@ function coverArtQueryParams(username: string, password: string, id: string, siz
     u: username,
     t: token,
     s: salt,
-    v: '1.16.1',
+    v: SUBSONIC_API_VERSION,
     c: SUBSONIC_CLIENT,
     f: 'json',
   });
@@ -36,7 +36,7 @@ function streamUrlFromProfile(
     u: username,
     t: token,
     s: salt,
-    v: '1.16.1',
+    v: SUBSONIC_API_VERSION,
     c: SUBSONIC_CLIENT,
     f: 'json',
   });
@@ -116,7 +116,7 @@ export function buildDownloadUrl(id: string): string {
   const p = new URLSearchParams({
     id,
     u: server?.username ?? '',
-    t: token, s: salt, v: '1.16.1', c: SUBSONIC_CLIENT, f: 'json',
+    t: token, s: salt, v: SUBSONIC_API_VERSION, c: SUBSONIC_CLIENT, f: 'json',
   });
   return `${baseUrl}/rest/download.view?${p.toString()}`;
 }

src/utils/server/serverFingerprint.ts

@@ -15,7 +15,7 @@
  */
 
 import md5 from 'md5';
-import { apiWithCredentials, restBaseFromUrl, secureRandomSalt, SUBSONIC_CLIENT } from '../../api/subsonicClient';
+import { apiWithCredentials, restBaseFromUrl, secureRandomSalt, SUBSONIC_CLIENT, SUBSONIC_API_VERSION } from '../../api/subsonicClient';
 import type { ServerProfile } from '../../store/authStoreTypes';
 import { allNormalizedAddresses } from './serverEndpoint';
 
@@ -64,7 +64,7 @@ async function fetchPingFingerprint(
     u: username,
     t: token,
     s: salt,
-    v: '1.16.1',
+    v: SUBSONIC_API_VERSION,
     c: SUBSONIC_CLIENT,
     f: 'json',
   });