feat: Preset reset on Default, expanded fields & cross-modal nav

- Reset presets to system defaults when "Default" is clicked in Configure Models (resetGlobalPresets() + backend reset action)
- Expand preset fields to full config parity: ctx_length, ctx_history/ctx_input, vision, rate limits, kwargs, browser_http_headers
- Extract _normalizePresets() helper inside store to eliminate duplication
- Hide "Edit Presets" link when Per-Chat Override is disabled
- Add Settings + API Keys navigation buttons to Model Presets page
- Add dynamic import guard for plugin-settings-store.js in cross-modal navigation buttons
- Rename model switcher label from "Default" to "Default LLM"

plugins/_model_config/api/model_presets.py

@@ -17,4 +17,8 @@ class ModelPresets(ApiHandler):
             model_config.save_presets(presets)
             return {"ok": True, "presets": presets}
 
+        elif action == "reset":
+            presets = model_config.reset_presets()
+            return {"ok": True, "presets": presets}
+
         return Response(status=400, response=f"Unknown action: {action}")