fix(goals): apply CodeRabbit findings

- Switch the goal_accounts → accounts FK from on_delete: :cascade to
  :restrict. `Goal#must_have_at_least_one_linked_account` is enforced
  at write time; the cascade let a raw DELETE silently orphan a Goal
  whose only link pointed at the deleted account. Normal Rails
  Account#destroy still cleans up via `dependent: :destroy`, but the
  restrict guarantees the DB rejects any path that bypasses the
  association.
- projection_payload: required_monthly is now monthly_target_amount&.to_f
  so open-ended (no-target-date) goals serialize required_monthly: null
  instead of 0, matching the absence of a required pace.
- index page + sidebar nav-rail dot now read the Beta label via
  t("shared.beta") (and a new shared.beta locale key) instead of the
  hardcoded "Beta" literal.
- _status_callout uses the view-helper t(...) instead of I18n.t(...)
  for the status label so it follows the same convention as the rest
  of the goals views.
- goal_projection_chart: read the computed style before stamping
  position: relative so a stylesheet-defined position (fixed/sticky/
  absolute) isn't clobbered.
- preview-deploy: add `set -euo pipefail` around the wrangler
  container lookup so a curl/jq failure fails the job instead of
  producing an empty CONTAINER_ID and silently skipping cleanup.

app/javascript/controllers/goal_projection_chart_controller.js

@@ -433,7 +433,12 @@ export default class extends Controller {
       .attr("pointer-events", "none")
       .style("display", "none");
 
-    if (root.style.position !== "absolute") root.style.position = "relative";
+    // Only promote root to a positioned ancestor when it currently has no
+    // positioning context. Inline checks against `root.style.position`
+    // miss positions set via CSS (the inline style is empty), so we'd
+    // clobber a stylesheet `position: fixed/sticky/absolute` with our
+    // own `relative`. Read the computed style instead.
+    if (getComputedStyle(root).position === "static") root.style.position = "relative";
     const tooltip = document.createElement("div");
     tooltip.style.cssText = "position:absolute;pointer-events:none;display:none;background:var(--color-gray-900);color:var(--color-white);font-size:12px;line-height:1.35;padding:6px 8px;border-radius:6px;white-space:nowrap;z-index:5;box-shadow:0 2px 8px rgba(0,0,0,0.15);";
     root.appendChild(tooltip);