webui: load the model selected via ?model= when ?load=true (#26707)

* webui: load the model selected via ?model=

Opening the WebUI with ?model= selects the model but doesn't load it. The load only starts when you send your first message, so you wait for it then.

This loads it as soon as the page opens, while you're still typing your prompt. It's what the model dropdown already does, and it isn't awaited, so the UI still works while the model loads.

This is the path the Llama macOS app uses to open the WebUI, so it's a common way in.

* webui: gate the load behind ?load=true

Loading on landing is opt-in, so a plain ?model= link behaves as before and doesn't allocate memory on its own.

* webui: name the chat URL params

Collects the query params the chat routes read into a URL_PARAMS constant, instead of repeating the literals across three files. NEW_CHAT_PARAM folds into it.

tools/ui/src/lib/components/app/dialogs/DialogModelNotAvailable.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import { ICON_CLASS_DEFAULT } from '$lib/constants/css-classes';
+	import { URL_PARAMS } from '$lib/constants';
 	import * as AlertDialog from '$lib/components/ui/alert-dialog';
 	import { AlertTriangle, ArrowRight } from '@lucide/svelte';
 	import { goto } from '$app/navigation';
@@ -22,7 +23,7 @@
 	function handleSelectModel(model: string) {
 		// Build URL with selected model, preserving other params
 		const url = new URL(page.url);
-		url.searchParams.set('model', model);
+		url.searchParams.set(URL_PARAMS.MODEL, model);
 
 		handleOpenChange(false);
 		goto(url.toString());