admin: surface CMF worker threads + GPU toggle in Settings

Expose the two speed knobs in the /admin Settings panel instead of TOML-only:
- [cmf].gpu (new) -> exports CMF_GPU=1 to the managed cortiq serve (Metal/wgpu).
  On Apple Silicon decode is memory-bandwidth bound so this is ~neutral for
  generation (helps prefill); off by default.
- [cmf].threads (added earlier) now has a UI number input.

- config.rs: CmfCfg.gpu bool (serde default false).
- cmf_runtime.rs: spawn_serve sets CMF_GPU=1 when gpu is on.
- admin/mod.rs: put_settings flags needs_restart on threads/gpu change (the
  server is spawned once at startup with those env vars).
- settings.js: threads input + gpu checkbox + save payload + speed hint.
- i18n en/ru: threads/gpu/speedNote keys (other locales fall back to en, as the
  whole cmf section already does there).

src/cmf_runtime.rs

@@ -167,6 +167,11 @@ fn spawn_serve(cfg: &CmfCfg) -> Result<Child, String> {
     if cfg.threads > 1 {
         cmd.env("CMF_THREADS", cfg.threads.to_string());
     }
+    // Offload large matvecs to the GPU (Metal on macOS). Memory-bandwidth-bound
+    // on unified memory, so mostly a prefill win; opt-in.
+    if cfg.gpu {
+        cmd.env("CMF_GPU", "1");
+    }
     cmd.stdout(Stdio::null())
         .stderr(Stdio::null())
         .kill_on_drop(true)