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/config.rs

@@ -116,6 +116,11 @@ pub struct CmfCfg {
     /// (measured ~2.8x faster than serial on a 15B model). Default 8.
     #[serde(default = "default_cmf_threads")]
     pub threads: usize,
+    /// Offload large matvecs to the GPU (exports CMF_GPU=1). On Apple Silicon the
+    /// decode loop is memory-bandwidth bound, so this is ~neutral for generation
+    /// (helps prefill of long prompts). Off by default.
+    #[serde(default)]
+    pub gpu: bool,
 }
 impl Default for CmfCfg {
     fn default() -> Self {
@@ -135,6 +140,7 @@ impl Default for CmfCfg {
             model_id: default_cmf_model_id(),
             local_only: false,
             threads: default_cmf_threads(),
+            gpu: false,
         }
     }
 }