Use a lighter icon for Custom Models

Boxes is the densest glyph in the settings nav by some way — nine
elements and 43 path commands against a median of about twelve. Three
cubes with their internal facets inside 20px puts the strokes almost on
top of each other, so at the nav's 1.75 weight it renders as a darker
blob than the icons above and below it, even though the stroke width is
identical. Blocks sits at the median, matching Sources, Tools and Teams,
and stays distinct from the database glyph two rows up.

frontend/src/navigation/sections.ts

@@ -1,7 +1,7 @@
 import {
   BarChart3,
+  Blocks,
   CalendarClock,
-  Boxes,
   ChartNoAxesColumn,
   Database,
   FileClock,
@@ -125,7 +125,7 @@ export const SETTINGS_SECTION: Section = {
           key: 'customModels',
           path: '/settings/custom-models',
           labelKey: 'settings.customModels.label',
-          icon: Boxes,
+          icon: Blocks,
         },
         {
           key: 'teams',