feat(savings): add savings goals

Adds a standalone Savings goals feature: a piggy-bank style tracker that
lets a family set a target, link one or more Depository accounts as
funding sources, and log manual contributions over time. Supersedes #1569
(closed) — same intent, redesigned per reviewer + Discord feedback.

What this adds:

- New `/savings_goals` sidebar entry (piggy-bank icon) with index, show,
  state-filtered tabs (all/active/paused/completed/archived), and a
  2-step modal stepper for creation (Identity → Review).
- Multi-account funding via a `SavingsGoalAccount` join: a goal requires
  ≥1 linked Depository account (checking/savings/HSA/CD/money-market),
  and all linked accounts must share the goal's currency.
- Tracker balance model: goal balance = SUM(contributions.amount). No
  auto-flow from account balances. Contributions are pure logical
  records and don't move money between accounts.
- Manual contributions modal scoped to the goal's linked accounts.
  Initial contributions seeded at creation can't be deleted; manual
  ones can.
- AASM lifecycle: active / paused / completed / archived.
  Hard-delete only after archive.
- Status pills (On track / Behind / Reached / No date) derived from
  pace vs target_date.
- AI Assistant tool `create_savings_goal` lets the sidebar chat create
  a goal end-to-end from a natural-language prompt; soft errors carry
  the available-accounts list back to the LLM (mirrors the existing
  `import_bank_statement` pattern).
- Family-scoped throughout (`Current.family`-only access, account
  family-scoping enforced both in controllers and the AI tool).
- Demo data seed wires up 4 sample goals across the Depository accounts.

Intentionally out of scope (separate PRs / v1.1):

- Auto-fund from budget surplus + Sidekiq cron + budget-show card.
- Dashboard "Savings goals" widget.
- "Behind pace" projection chart on the detail page.
- `evaluate_savings_goal_feasibility` LLM tool (level-setting before
  create_savings_goal).
- Spend-less goals inside Budgets.
- Family-member-private goals (deferred investigation).

test/controllers/savings_contributions_controller_test.rb

@@ -0,0 +1,58 @@
+require "test_helper"
+
+class SavingsContributionsControllerTest < ActionDispatch::IntegrationTest
+  setup do
+    sign_in users(:family_admin)
+    @goal = savings_goals(:vacation_italy)
+    @depository = accounts(:depository)
+    ensure_tailwind_build
+  end
+
+  test "new renders the modal form" do
+    get new_savings_goal_contribution_url(@goal)
+    assert_response :success
+  end
+
+  test "create saves a manual contribution" do
+    assert_difference -> { @goal.savings_contributions.count } => 1 do
+      post savings_goal_contributions_url(@goal), params: {
+        savings_contribution: {
+          amount: "100",
+          contributed_at: Date.current.iso8601,
+          notes: ""
+        },
+        savings_contribution_account_id: @depository.id
+      }.merge(savings_contribution: { account_id: @depository.id, amount: "100", contributed_at: Date.current.iso8601 })
+    end
+
+    assert_redirected_to savings_goal_path(@goal)
+    contribution = @goal.savings_contributions.order(created_at: :desc).first
+    assert_equal "manual", contribution.source
+    assert_equal @depository, contribution.account
+  end
+
+  test "create rejects contribution from non-linked account" do
+    unlinked = Account.create!(family: @goal.family, accountable: Depository.new, name: "Unlinked", currency: "USD", balance: 100)
+    assert_no_difference "@goal.savings_contributions.count" do
+      post savings_goal_contributions_url(@goal), params: {
+        savings_contribution: { amount: "10", contributed_at: Date.current.iso8601, account_id: unlinked.id }
+      }
+    end
+    assert_response :unprocessable_entity
+  end
+
+  test "destroy manual contribution removes it" do
+    manual = savings_contributions(:vacation_italy_manual)
+    assert_difference "SavingsContribution.count", -1 do
+      delete savings_goal_contribution_url(@goal, manual)
+    end
+  end
+
+  test "destroy initial contribution is blocked" do
+    initial = savings_contributions(:vacation_italy_initial)
+    assert_no_difference "SavingsContribution.count" do
+      delete savings_goal_contribution_url(@goal, initial)
+    end
+    assert_redirected_to savings_goal_path(@goal)
+  end
+end