fix(downloads): route unknown checkpoint baseModels to diffusion models by default

CivitAI labels new DiT architectures (MiniMax H3, future Flux/Wan/Qwen
variants) as model.type "Checkpoint" with plain "Model" file entries,
so the DIFFUSION_MODEL_BASE_MODELS allowlist could never keep up and
such downloads were mis-routed to the checkpoint roots (e.g. model
2877206 / version 3374439). The set of true full-checkpoint families is
closed, so the baseModel fallback is inverted:

1. file type UNet/Diffusion Model -> unet (unchanged)
2. baseModel in DIFFUSION_MODEL_BASE_MODELS (now incl. MiniMax H3) -> unet
3. baseModel in new CHECKPOINT_BASE_MODELS (SD 1.x/2.x/3.x, SDXL, Pony,
   Illustrious, NoobAI) -> checkpoint
4. unknown/empty baseModel -> new unknown_base_model_routing setting,
   defaulting to diffusion models

The setting is exposed under Settings > Downloads, validated in
SettingsManager, and threaded into both the download manager and the
download routing endpoint so they keep agreeing.

static/js/managers/SettingsManager.js

@@ -1053,6 +1053,11 @@ export class SettingsManager {
             downloadBackendSelect.value = state.global.settings.download_backend || 'python';
         }
 
+        const unknownBaseModelRoutingSelect = document.getElementById('unknownBaseModelRouting');
+        if (unknownBaseModelRoutingSelect) {
+            unknownBaseModelRoutingSelect.value = state.global.settings.unknown_base_model_routing || 'diffusion_model';
+        }
+
         const aria2cPathInput = document.getElementById('aria2cPath');
         if (aria2cPathInput) {
             aria2cPathInput.value = state.global.settings.aria2c_path || '';

static/js/state/index.js

@@ -60,6 +60,7 @@ const DEFAULT_SETTINGS_BASE = Object.freeze({
     metadata_refresh_skip_paths: [],
     skip_previously_downloaded_model_versions: false,
     download_skip_base_models: [],
+    unknown_base_model_routing: 'diffusion_model',
     backup_auto_enabled: true,
     backup_retention_count: 5,
     sidecar_storage_mode: 'alongside',