feat(whisper): add Distil-Whisper Small and Large v3 as first-class models

Two new registry entries (crates/core/src/model_registry.rs):
- whisper-distil-small-en — 336 MB, ~6× faster than whisper-small-en
- whisper-distil-large-v3 — 1.55 GB, near large-v3 accuracy at medium size

Both are whisper.cpp-compatible GGML binaries hosted on HF by the
distil-whisper org; no runtime change, just wider model choice. English-only
by design (matches upstream Distil-Whisper).

The Settings model picker widens to six options — Tiny, Base, Small,
Distil-S, Medium, Distil-L — ordered roughly by accuracy. Download/load
commands now take the resolved model id (whisper-distil-*) instead of the
lowercased label, so the frontend owns the label↔id mapping.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/pages/SettingsPage.svelte

@@ -337,7 +337,9 @@
       Tiny: "whisper-tiny-en",
       Base: "whisper-base-en",
       Small: "whisper-small-en",
+      "Distil-S": "whisper-distil-small-en",
       Medium: "whisper-medium-en",
+      "Distil-L": "whisper-distil-large-v3",
     };
     return map[size] || "whisper-base-en";
   }
@@ -614,7 +616,7 @@
   }
 
   async function loadSelectedModel() {
-    const size = settings.modelSize.toLowerCase();
+    const size = whisperModelId(settings.modelSize);
     engineStatus = "Loading...";
     engineOk = false;
     try {
@@ -650,7 +652,9 @@
     Tiny: "~75MB · fastest, lower accuracy",
     Base: "~150MB · balanced for most use",
     Small: "~500MB · noticeably more accurate",
-    Medium: "~1.5GB · best quality, slower",
+    "Distil-S": "~336MB · small-level accuracy at ~6× the speed",
+    Medium: "~1.5GB · best Whisper accuracy, slower",
+    "Distil-L": "~1.55GB · near-large accuracy at ~6× the speed",
   };
 
   async function downloadParakeet() {
@@ -1004,7 +1008,7 @@
             {#if settings.engine === "whisper"}
               <div class="mb-6">
                 <p class="text-[10px] font-medium text-text-tertiary uppercase tracking-wider mb-2">Whisper Model</p>
-                <SegmentedButton options={["Tiny", "Base", "Small", "Medium"]} bind:value={settings.modelSize} />
+                <SegmentedButton options={["Tiny", "Base", "Small", "Distil-S", "Medium", "Distil-L"]} bind:value={settings.modelSize} />
                 <p class="text-[11px] text-text-tertiary mt-2">{modelDescriptions[settings.modelSize]}</p>
 
                 <div class="flex items-center gap-2 mt-3">
@@ -1022,12 +1026,12 @@
                       class="text-[11px] text-text-tertiary hover:text-accent"
                       onclick={loadSelectedModel}
                     >Load model</button>
-                  {:else if downloadingModel === settings.modelSize.toLowerCase()}
+                  {:else if downloadingModel === whisperModelId(settings.modelSize)}
                     <span class="text-[11px] text-warning">{downloadProgress}% downloading...</span>
                   {:else}
                     <button
                       class="text-[11px] text-accent hover:text-accent-hover"
-                      onclick={() => downloadModel(settings.modelSize.toLowerCase())}
+                      onclick={() => downloadModel(whisperModelId(settings.modelSize))}
                     >Download {settings.modelSize}</button>
                   {/if}
                 </div>