fix(ui): enable custom parameters in user settings and admin model settings (#25200)

Add missing custom={true} prop to AdvancedParams in General.svelte and
ModelSettingsModal.svelte so the 'Add Custom Parameter' option appears
consistently across all advanced parameter surfaces.

Also forward custom_params in the General.svelte save handler so custom
parameters are persisted instead of silently dropped on save.

src/lib/components/admin/Settings/Models/ModelSettingsModal.svelte

@@ -366,7 +366,7 @@
 
 											{#if showDefaultParams}
 												<div class="mt-2">
-													<AdvancedParams admin={true} bind:params={defaultParams} />
+													<AdvancedParams admin={true} custom={true} bind:params={defaultParams} />
 												</div>
 											{/if}
 										</div>

src/lib/components/chat/Settings/General.svelte

@@ -109,7 +109,10 @@
 				num_gpu: params.num_gpu !== null ? params.num_gpu : undefined,
 				think: params.think !== null ? params.think : undefined,
 				keep_alive: params.keep_alive !== null ? params.keep_alive : undefined,
-				format: params.format !== null ? params.format : undefined
+				format: params.format !== null ? params.format : undefined,
+				...(params.custom_params && Object.keys(params.custom_params).length > 0
+					? { custom_params: params.custom_params }
+					: {})
 			}
 		});
 		dispatch('save');
@@ -324,7 +327,7 @@
 				</div>
 
 				{#if showAdvanced}
-					<AdvancedParams admin={$user?.role === 'admin'} bind:params />
+					<AdvancedParams admin={$user?.role === 'admin'} custom={true} bind:params />
 				{/if}
 			</div>
 		{/if}