Add Account Billing section + Stripe Customer Portal CTA (#33) (#71)

## Summary
- Closes [#33](#33)
- Add Billing glass cards on `/account/` (`Account2`): current plan summary + billing history from `GET /finance/invoices/` (and payments fallback)
- **Manage subscription** CTA → `POST /finance/portal/` → redirect to Stripe hosted Customer Portal (return URL `/account/`)
- Empty / unpaid users get **Complete payment** → Checkout instead of a dead portal button
- Visible errors for list/portal failures; fire `BILLING_PORTAL_OPENED`
- Depends on backend portal endpoint: [chat_backend#35](ai_ml_operations/chat_backend#35)

## Test plan
- [ ] `npm test -- --watchAll=false --testPathPattern='(BillingSection|finance\\.test)'`
- [ ] Signed-in user with paid invoices: plan + history render; Manage subscription redirects to portal URL
- [ ] User with no invoices: empty copy + Complete payment starts Checkout
- [ ] Portal/list API errors show alert text (no silent fail)
- [ ] Return from Stripe portal lands on `/account/`
- [ ] No card fields collected in-appReviewed-on: #71

llm-fe/ANALYTICS.md

@@ -20,7 +20,7 @@ Page views: always via `Tracker` / `tracker.js` on prod + beta.
 | Conversation Created | `CONVERSATION_CREATED` | MessageContext | `{ conversationId }` |
 | Message Sent | `MESSAGE_SENT` | AsyncDashboard2 | `{ hasConversation, hasAttachment }` |
 | ToS Acknowledged | `TOS_ACKNOWLEDGED` | TermsOfService | — |
-| Billing Portal Opened | `BILLING_PORTAL_OPENED` | *(pending #33 Account portal CTA)* | — |
+| Billing Portal Opened | `BILLING_PORTAL_OPENED` | BillingSection (Account) | — |
 
 ## Identify