feat: #3620064 Add Canvas Icon Picker and set the default icon packs

Issue: https://www.drupal.org/project/varbase_admin_base/issues/3620064

Installs canvas_icon_picker beside ui_icons, and sets the two icon packs every Vartheme BS5 theme declares in its <theme>.icons.yml.

Declaring it here rather than in each site template means Varbase Starter, Educare, Horizon Aid and RightUp all inherit it from one file. Varbase Starter drops its own declaration in a companion issue; do not merge that one until this has been released, or Starter loses the picker in the gap.

~1.0.3 is deliberate: 1.0.3 is the first release carrying drupal.org/node/3619510 and drupal.org/node/3619785, which are what let a component prop keep its curated enum and still get the picker.

Verified on two fresh Varbase 11 builds, Educare and Horizon Aid, each installing green with the module coming from this recipe alone: allowed_packs reads back correctly on both, every Canvas page returns 200 anonymously, the picker endpoints return 403 anonymously, and neither site logs anything at Error or worse.

One commit, 13 added lines and nothing removed against 1.0.x.

AI-Generated: Yes

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