fix(my-account): v2 demo subscriptions polish + expired variant

[thomasguillot]

Polish pass on the v2-demo subscriptions list against Figma 2636:46117, plus a small fix to the detail-page expired variant. Targets prototype/my-account-demo (umbrella tracker #4679). Devlog rows tagged Subscriptions list polish capture the why for each call.

Summary

Subscriptions list (Figma 2636:46117):

• Section h2 __font--l __font--bold → __font--m __font--bold (Figma 20/32; matches the typography migration donations + newsletters got earlier today).
• Plan-name + price __font--m __font--bold → __font--s __font--bold (Figma 16/24 SemiBold).
• Subtext gains __font--normal for explicit weight parity with donations.
• Outer wrapper __stack--gap-9 (48px) → __stack--gap-11 (64px). Side benefit: satisfies v1's :not(.__stack--gap-11) section + section { margin-top: spacer-11 } exclusion so the second section's 64px margin-top stops compounding on top of the gap (prior cascade was producing ~112px between Active and Previous).
• New .newspack-my-account-v2-demo__separator primitive — 1px × 1em vertical hairline — replaces the literal | glyph between plan name and price (Figma renders this as a 1px × 16px #ddd div, not a character). Page-agnostic so future cards / headers can reuse without per-page rule duplication.

Sidebar:

• "Payment methods" → "Payment information". Matches Figma + v1's payment-information.php shell. Slug stays payment-methods (URL routing unchanged); only the display label changes.

Subscription detail (/sub-expired/):

• $is_expired was an undeclared fall-through — the page rendered with no header badge and no actions any time a reader clicked an Expired card in Previous.
• EXPIRED badge added (mirrors cancelled, same __badge--error).
• Renew subscription button + renew-subscription modal partial extended to the expired branch.
• File docblock updated to list all six functional variants.

Subscription detail audit (Figma 2636:46149): came back clean — existing scoped CSS already pins header h2 to --font-size-m, kv tables to --font-size-s, Billing history h2 to --font-size-s. No further pinning needed this pass.

Test plan

• /my-account/subscriptions/?my-account-v2-demo=1 — section titles drop a notch, "Member · £71.16 / year" reads at 16/24 SemiBold, the gap between Active and Previous opens to 64px (no more compounded margin), the | glyph is replaced with a thin gray vertical hairline.
• Sidebar reads "Payment information" (last item).
• /my-account/subscriptions/sub-expired/?my-account-v2-demo=1 — header shows EXPIRED badge next to product name + a Renew subscription button. Clicking Renew opens the renew-subscription modal.
• /my-account/subscriptions/sub-cancelled/?my-account-v2-demo=1 — unchanged (Cancelled badge + Renew, sanity check that the elseif didn't break the cancelled branch).
• /my-account/subscriptions/sub-001/?my-account-v2-demo=1 — detail page renders unchanged (audit confirmed no changes needed).
• Non-demo admin and anonymous-with-flag both see v1 unchanged.
• Lint clean (:php, :scss); build green.