…cation pages (closes #314)

Introduces the Archera Insurance CTA at the two high-intent moments
where a user is about to commit money, plus two education pages that
explain what Archera Insurance is and how the integration works.

**New module — frontend/src/archera.ts**
- `renderArcheraCTA()`: returns a small, muted <p> with an inline
  <button> that reads "Worried about committing? Archera Insurance
  underwrites commitment-overuse — learn how it works →". Placed
  below the approval note in the Purchase modal, and before the
  Save/Cancel row in Plan-creation modals.
- `openArcheraPage('what-is-archera' | 'how-it-works')`: renders a
  full-viewport overlay panel using DOM methods only (no innerHTML).
  Doesn't touch SPA routing state or the URL.
- `closeArcheraPage()`: hides + clears the overlay.
- `ARCHERA_SIGNUP_URL = 'https://archera.ai/signup?mode=cudly'` for
  attribution-tagged referral links.

**Page A — "What is Archera Insurance?"**
Plain-language explainer: commitment-overuse insurance, how Archera
earns (sharing-of-savings), when it makes sense, what CUDly's role is.
Cross-links to Page B. Signup link at bottom (target=_blank,
rel="noopener noreferrer").

**Page B — "How the CUDly ↔ Archera integration works"**
Step-by-step: sign up → Archera generates pre-filled IaC for AWS/GCP
(or OAuth consent for Azure) → Archera ingests cost data → purchase
commitments normally through CUDly. Disclaimers: Archera is
third-party, no credentials stored in CUDly. Cross-links to Page A.
Signup link at bottom.

TODO(@cristim) markers flag all copy that needs final review.

**Modal integration**
- `recommendations.ts / openPurchaseModal`: appends `renderArcheraCTA()`
  inside `#purchase-details`, below the approval note.
- `plans.ts / openCreatePlanModal + openNewPlanModal`: calls
  `injectPlanModalCTA()` (guarded against duplication with an id
  check) to insert the CTA before `.modal-buttons` in `#plan-form`.

**CSS (styles/modals.css)**: `.archera-cta`, `.archera-cta-link`,
`#archera-page-container`, `.archera-page-inner`, `.archera-page-back`,
`.archera-signup-block`, `.archera-signup-btn`, `.archera-signup-note`.

**HTML (index.html)**: adds `<div id="archera-page-container"
role="region" aria-label="Archera Insurance information">` (hidden
by default) before the modal stack.

**Tests (archera.test.ts)**: 26 assertions covering renderArcheraCTA
structure, page A/B content + crosslinks + signup link attributes,
closeArcheraPage, and CTA injection in both Purchase and Plan modals.

Nothing in backend/, cmd/, internal/, iac/, terraform/, or scripts/ is
touched. This is a frontend-only change. Supersedes PR #310 and PR #315
(closed without merging; earlier iterations bundled IAM/IaC which was
rejected by Archera-side review — see issue #314 for history).

Restructures the Archera Insurance flow into a post-action offer modal,
collapses the previously split education pages into one, and tightens
the copy.

Frontend changes:
- Replace the inline "Insure this {commitment,plan} with Archera →" CTA
  inside the Purchase and Plan-creation modals with a new post-action
  offer modal (openArcheraOfferModal). The modal pops after a successful
  Send-for-Approval submission (app.ts handleExecutePurchase +
  handleFanOutExecute) or plan creation (plans.ts savePlan). The user
  sees title + lead + brief disclosure, then a collapsed "Learn more"
  drop-down, then two equal-weight actions: "No thanks" and "Sign up at
  Archera →" (opens archera.ai in a new tab).
- Merge the previously separate "What is Archera Insurance?" (Page A)
  and "How the integration works" (Page B) overlays into a single
  education page (buildArcheraPage). Both ARCHERA_PAGE_A_PATH and
  ARCHERA_PAGE_B_PATH still deep-link to the merged page (PAGE_B kept
  for backward compatibility with already-delivered email links).
- "Learn more" is a native <details>/<summary> that expands the full
  education content inline within the offer modal. Summary text on the
  left, chevron on the right (rotates on open). Body inherits .archera-
  page-inner typography for visual parity with the standalone page.
- Education content tightened: dedupe the lead-paragraph + "Why is CUDly
  telling me about this?" overlap, drop the four-bullet Disclaimers
  section in favour of a single closing "Full disclosure:" paragraph
  that absorbs the legal/scope points. Drop the "All Upfront" qualifier
  from "When it makes sense" and reframe the second use case from
  "finance team requires a safety net" to user-perspective "you want to
  be covered in case your usage drops".
- "How it works" intro now explicitly states the 7-day enrollment
  window from each purchase.
- Premium note clarifies the insurance fee is paid to Archera and the
  cloud commitment bought through CUDly is unaffected (same price,
  same billing) — replaces the earlier sharing-of-savings framing.
- Add aria-haspopup="dialog" + aria-expanded toggling on the (now-
  removed) Columns popover trigger to expose dialog state to assistive
  tech. (No-op for the removed inline CTAs; this change is the modal's
  a11y surface.)
- Drop the inline scripts' em-dashes from JSDoc + user-facing strings.

Tests:
- archera.test.ts rewritten: drops renderArcheraCTA + page A/B + plan-
  modal-CTA + purchase-modal-CTA describe blocks (those surfaces are
  gone). Adds openArcheraOfferModal describes covering visibility,
  context-dependent headlines, signup link attributes, No-thanks +
  backdrop + ESC close, Learn-more collapsible expand, re-open replace
  semantics. handleArcheraDeeplink test now verifies both legacy and
  current URL paths open the same merged page. 1605 frontend tests
  pass.

…alog

Address two CR findings on PR #336:

1. **Stale keydown listener leak**: openArcheraOfferModal added a
   document-level keydown handler (ESC closes) and stored its cleanup in
   `_offerModalCleanup` on every call, but didn't first invoke the prior
   cleanup. A re-open without an explicit close in between leaked the
   previous handler. Fix: call `_offerModalCleanup()` (if set) at the top
   of openArcheraOfferModal before attaching the new handler. Idempotent
   — no-op when no prior open is in flight.

2. **No focus trap on the dialog panel**: the panel declares
   `role="dialog" aria-modal="true"` but Tab / Shift+Tab let keyboard
   users escape into the background app. That's an a11y regression for
   an "aria-modal=true" element. Fix: attach a panel-level keydown
   handler that intercepts Tab — Tab on last focusable wraps to first,
   Shift+Tab on first wraps to last. Focusables are re-queried on each
   Tab press (not cached at modal-build time) so the Tab traversal
   picks up the additional focusables that the <details> drop-down
   expands when opened. Trap handler is torn down alongside the ESC
   handler by the merged `_offerModalCleanup` closure.

Tests in archera.test.ts:
- `re-opening tears down the previous open's ESC listener (no leak)`
  spies on document.addEventListener / removeEventListener and asserts
  the second open both adds AND removes one keydown handler.
- `traps Tab focus inside the panel (Tab from last → first)`
  focuses the last focusable, dispatches Tab, asserts focus wraps to
  the first and the event was preventDefaulted.
- `traps Shift+Tab focus inside the panel (Shift+Tab from first → last)`
  the mirror case.

1608 frontend tests pass (3 new).

…to feat/issue-314-archera-modal-cta

# Conflicts:
#	frontend/src/plans.ts

…to feat/issue-314-archera-modal-cta