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).

app/javascript/controllers/savings_goal_stepper_controller.js

@@ -0,0 +1,124 @@
+import { Controller } from "@hotwired/stimulus";
+
+// 2-step modal stepper for creating a savings goal.
+//
+// Single <form> with two panels. Step 1 collects identity (name, amount,
+// date, color, notes). Step 2 collects ≥1 linked depository accounts and
+// optionally an initial contribution. Submit button stays disabled until at
+// least one linked account is selected. Step state lives entirely in the
+// DOM — no half-records.
+export default class extends Controller {
+  static targets = [
+    "step1Panel",
+    "step2Panel",
+    "step1Indicator",
+    "step2Indicator",
+    "step1Field",
+    "nameField",
+    "targetAmountField",
+    "linkedAccountCheckbox",
+    "initialContributionAmount",
+    "initialContributionAccountSelect",
+    "reviewPanel",
+    "reviewName",
+    "reviewAmount",
+    "reviewDate",
+    "reviewAccounts",
+    "submitButton",
+  ];
+
+  next(event) {
+    event?.preventDefault?.();
+    if (!this.validateStep1()) return;
+
+    this.step1PanelTarget.classList.add("hidden");
+    this.step2PanelTarget.classList.remove("hidden");
+    this.markStepActive(2);
+    this.updateReview();
+    this.refreshSubmitState();
+  }
+
+  back(event) {
+    event?.preventDefault?.();
+    this.step2PanelTarget.classList.add("hidden");
+    this.step1PanelTarget.classList.remove("hidden");
+    this.markStepActive(1);
+  }
+
+  linkedAccountChanged() {
+    this.refreshAccountSelect();
+    this.refreshSubmitState();
+    this.updateReview();
+  }
+
+  validateStep1() {
+    let ok = true;
+    this.step1FieldTargets.forEach((field) => {
+      if (!field.checkValidity()) {
+        field.reportValidity();
+        ok = false;
+      }
+    });
+    return ok;
+  }
+
+  refreshSubmitState() {
+    const anyChecked = this.linkedAccountCheckboxTargets.some((cb) => cb.checked);
+    this.submitButtonTarget.disabled = !anyChecked;
+  }
+
+  refreshAccountSelect() {
+    if (!this.hasInitialContributionAccountSelectTarget) return;
+
+    const select = this.initialContributionAccountSelectTarget;
+    const previous = select.value;
+    select.innerHTML = "";
+    const blank = document.createElement("option");
+    blank.value = "";
+    blank.textContent = select.dataset.blankLabel || "—";
+    select.appendChild(blank);
+
+    this.linkedAccountCheckboxTargets
+      .filter((cb) => cb.checked)
+      .forEach((cb) => {
+        const opt = document.createElement("option");
+        opt.value = cb.value;
+        opt.textContent = cb.dataset.accountName || cb.value;
+        select.appendChild(opt);
+      });
+
+    if ([...select.options].some((o) => o.value === previous)) {
+      select.value = previous;
+    }
+  }
+
+  updateReview() {
+    if (!this.hasReviewPanelTarget) return;
+
+    if (this.hasReviewNameTarget && this.hasNameFieldTarget) {
+      this.reviewNameTarget.textContent = this.nameFieldTarget.value || "—";
+    }
+    if (this.hasReviewAmountTarget && this.hasTargetAmountFieldTarget) {
+      this.reviewAmountTarget.textContent = this.targetAmountFieldTarget.value || "—";
+    }
+    if (this.hasReviewDateTarget) {
+      const dateInput = this.element.querySelector('input[type="date"][name="savings_goal[target_date]"]');
+      this.reviewDateTarget.textContent = dateInput?.value || "—";
+    }
+    if (this.hasReviewAccountsTarget) {
+      const names = this.linkedAccountCheckboxTargets
+        .filter((cb) => cb.checked)
+        .map((cb) => cb.dataset.accountName || cb.value);
+      this.reviewAccountsTarget.textContent = names.length ? names.join(", ") : "—";
+    }
+  }
+
+  markStepActive(stepNumber) {
+    if (this.hasStep1IndicatorTarget) {
+      this.step1IndicatorTarget.classList.toggle("text-primary", stepNumber === 1);
+    }
+    if (this.hasStep2IndicatorTarget) {
+      this.step2IndicatorTarget.classList.toggle("text-primary", stepNumber === 2);
+    }
+  }
+}