fix(spotify): PATH-aware binary presence detection (bin/ or PATH) [whole-branch I3,m1]

rustPresent/goPresent and getBackendInfo used existsSync(findX()), which for a
bare PATH command name resolves against cwd, not $PATH — so a scoop/choco/cargo/
apt install was invisible and Spotify was gated off. Add a sync PATH-aware
resolveExecutable() + isLibrespotPresent()/isGoLibrespotPresent() in binary.ts
and route controller.ts and web/server.ts through them.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/web/server.ts

@@ -23,12 +23,9 @@ import { createSpotifyRouter } from "./api/spotify.js";
 import type { SpotifyOAuth } from "../music/spotify/spotify-oauth.js";
 import { resolveSpotifyBackendKind } from "../music/spotify/backend-select.js";
 import {
-  isGoLibrespotSupported,
-  findGoLibrespot,
-  isRustLibrespotSupported,
-  findLibrespot,
+  isGoLibrespotPresent,
+  isLibrespotPresent,
 } from "../music/spotify/binary.js";
-import { existsSync } from "node:fs";
 import { setupWebSocket } from "./websocket.js";
 import { createUserStore } from "../data/users.js";
 import { createSessionStore } from "../data/sessions.js";
@@ -156,10 +153,10 @@ export function createWebServer(options: WebServerOptions): WebServer {
         oauth: options.spotifyOAuth,
         logger,
         getBackendInfo: () => {
-          const goPresent =
-            isGoLibrespotSupported() && existsSync(findGoLibrespot());
-          const rustPresent =
-            isRustLibrespotSupported() && existsSync(findLibrespot());
+          // PATH-aware presence (Bug m1): a PATH-installed binary (bare name)
+          // is resolved against $PATH, not existsSync()'d against cwd.
+          const goPresent = isGoLibrespotPresent();
+          const rustPresent = isLibrespotPresent();
           const resolved = resolveSpotifyBackendKind(
             options.config.spotify.backend,
             goPresent,