feat(goals): extract shared color_icon_picker controller; add icon to goals; tinted avatar

User requested replacing the in-house color disclosure with the
categories color+icon popover. Done as a controller extraction so
categories and goals share one Stimulus controller (user's option:
"Extract a shared color_icon_picker_controller.js").

- `git mv` app/javascript/controllers/category_controller.js to
  color_icon_picker_controller.js. Categories form + color_avatar
  partial updated to use the new identifier (data-controller=
  "color-icon-picker", target/action selectors renamed).
- Goal model gains an icon column (migration
  20260511190000_add_icon_to_goals.rb) + ICONS = Category.icon_codes
  + inclusion validation. GoalsController permits :icon in
  goal_params + goal_update_params.
- Goals::AvatarComponent now renders icon when present (falls back to
  first-letter initial), and adopts the Categories tinted-bg + colored
  -content style (bg = `color-mix(in oklab, COLOR 10%, transparent)`,
  text/icon = COLOR). Matches the picker's live preview so what the
  user sees during selection equals the saved state.
- New goals/_color_picker.html.erb mirrors categories/_form's popover:
  avatar + pen overlay summary + popup with color row (+ rainbow
  custom-hex trigger) + icon grid. Pickr / contrast validation / auto-
  adjust all inherited from the shared controller.
- Stepper step 1 layout: drop the inline letter-avatar (data-goal-
  stepper-target="avatarPreview") in favour of the picker avatar next
  to the name input. Step 1's tail no longer renders a separate color
  partial. Edit form passes icons local through.

Verified live: new goal modal renders 11 color radios (10 presets +
custom) + 141 icon radios + pen-summary; categories form still
operational (no console errors) under the renamed controller.

db/migrate/20260511190000_add_icon_to_goals.rb

@@ -0,0 +1,5 @@
+class AddIconToGoals < ActiveRecord::Migration[7.2]
+  def change
+    add_column :goals, :icon, :string
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_05_11_100003) do
+ActiveRecord::Schema[7.2].define(version: 2026_05_11_190000) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -658,7 +658,7 @@ ActiveRecord::Schema[7.2].define(version: 2026_05_11_100003) do
     t.index ["goal_id", "contributed_at"], name: "index_goal_contributions_on_goal_id_and_contributed_at"
     t.index ["goal_id"], name: "index_goal_contributions_on_goal_id"
     t.check_constraint "amount > 0::numeric", name: "chk_savings_contributions_amount_positive"
-    t.check_constraint "source::text = ANY (ARRAY['manual'::character varying, 'initial'::character varying]::text[])", name: "chk_savings_contributions_source_enum"
+    t.check_constraint "source::text = ANY (ARRAY['manual'::character varying::text, 'initial'::character varying::text])", name: "chk_savings_contributions_source_enum"
   end
 
   create_table "goals", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|
@@ -672,6 +672,7 @@ ActiveRecord::Schema[7.2].define(version: 2026_05_11_100003) do
     t.string "state", default: "active", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.string "icon"
     t.index ["family_id", "state"], name: "index_goals_on_family_id_and_state"
     t.index ["family_id"], name: "index_goals_on_family_id"
     t.check_constraint "char_length(name::text) <= 255", name: "chk_savings_goals_name_length"