Merge origin/feat/goals-v2-architecture; reconcile beta→preview rename

Remote branch added a beta_gated_nav_item helper + 'Gating the main nav'
docs section. Main concurrently renamed the beta-features gate to
preview-features (concern, predicate, JSONB key, locale flash). Rename
the new helper / partial local / pill marker to match preview naming and
port the nav-gating docs into gating-a-preview-feature.md so the
improvement survives the rename.

Resolved conflicts:
- db/schema.rb: take the later schema version (2026_05_19_100000).
- docs/llm-guides/gating-a-beta-feature.md: accept main's deletion;
  port the 'Gating the main nav' section into the preview guide.

Renames carried through to keep the gate wired end-to-end:
- application_helper.rb: beta_gated_nav_item → preview_gated_nav_item;
  beta_features_enabled? → preview_features_enabled?; beta: → preview:.
- _nav_item.html.erb: beta: local → preview: local; shared.beta i18n
  key → shared.preview.
- application.html.erb: caller renamed to preview_gated_nav_item.
- goals/index.html.erb: pill label uses shared.preview.
- shared/en.yml: 'beta: Beta' → 'preview: Preview'.
- goals_controller, goal_pledges_controller: require_beta_features! →
  require_preview_features!.
- goals_controller_test, goal_pledges_controller_test: flip the
  preference key, flash matcher, and test names to 'preview'.

test/controllers/goal_pledges_controller_test.rb

@@ -2,13 +2,24 @@ require "test_helper"
 
 class GoalPledgesControllerTest < ActionDispatch::IntegrationTest
   setup do
-    sign_in users(:family_admin)
+    @user = users(:family_admin)
+    @user.update!(preferences: (@user.preferences || {}).merge("preview_features_enabled" => true))
+    sign_in @user
     @goal = goals(:vacation_italy)
     @account = accounts(:depository)
     @pledge = goal_pledges(:open_transfer)
     ensure_tailwind_build
   end
 
+  test "redirects users without preview access" do
+    @user.update!(preferences: (@user.preferences || {}).merge("preview_features_enabled" => false))
+
+    get new_goal_pledge_url(@goal), headers: { "Turbo-Frame" => "modal" }
+
+    assert_redirected_to root_path
+    assert_match(/preview/i, flash[:alert])
+  end
+
   test "new renders the pledge form inside a turbo frame" do
     get new_goal_pledge_url(@goal), headers: { "Turbo-Frame" => "modal" }
     assert_response :success