feat(frontend): surface 15% yearly savings in switch-to-yearly modal (#13095)

### Why / What / How

**Why:** The current in-app cycle-switch confirmation modal (Settings →
Subscription → cycle toggle → Yearly) only mentions the prorated charge
and the next yearly renewal amount. It does not surface the **15% yearly
discount**, the monthly-equivalent yearly price, or the apples-to-apples
annual comparison. This is a high-friction billing moment — Stripe
charges the prorated difference immediately on confirm — and the user
has no in-context reason to accept it. SECRT-2345 calls this out as
Urgent.

**What:** Rewrites the body and title of the `SwitchCycleDialog` for the
monthly→yearly direction so the savings, monthly-equivalent price, and
annual comparison are the first thing the user reads, before the
existing proration line. Title now names the tier (e.g. `Switch Pro to
yearly billing?` / `Switch Max to yearly billing?`).

**How:**
- `useYourPlanCard.ts` — added `getDialogTitle()` and rewrote the yearly
branch of `getDialogBody()`. Savings percent is **computed** from
`currentMonthlyCents * 12` vs `currentYearlyCents` rather than
hard-coded `15%`, so the copy stays accurate if backend pricing shifts.
- `SwitchCycleDialog.tsx` — accepts an optional `title` prop overriding
the generic `Switch billing to Yearly?` default.
- `YourPlanCard.tsx` — wires `cycleDialogTitle` through to the dialog.
- Falls back to the original generic copy when prices are missing (e.g.
`tier_costs_yearly` not in payload) or the tier label isn't meaningful
for the title (`NO_TIER`, `BUSINESS`). No behavior change for the
monthly direction or the tier-upgrade / tier-downgrade dialogs.

Stripe price IDs, tier mapping, and the proration flow are untouched —
this is a copy-only change on the existing yearly switch path.

<img width="676" height="308" alt="Screenshot 2026-05-13 at 7 57 37 AM"
src="https://github.com/user-attachments/assets/94afe427-18fb-40fa-9b03-a98bfe5d09e7"
/>

<img width="682" height="284" alt="Screenshot 2026-05-13 at 7 58 01 AM"
src="https://github.com/user-attachments/assets/ef8de562-a957-45f0-9c6b-473a830d2635"
/>


### Changes 🏗️

- New title for monthly→yearly: `Switch Pro to yearly billing?` /
`Switch Max to yearly billing?` (was: `Switch billing to Yearly?`).
- New body for monthly→yearly (Pro example):
> Save 15% with yearly billing. Pro is $42.50/month when billed yearly,
charged as $510.00/year instead of $600.00/year monthly. You'll be
charged the prorated difference immediately. After this period, your
plan renews yearly at $510.00.
- Max body equivalent uses `$272.00/month`, `$3,264.00/year`, `instead
of $3,840.00/year`.
- Savings percent computed from live prices (currently 15% for Pro and
Max).
- Existing tests updated; new test added for the Max yearly-switch copy.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] Pro monthly subscriber → Settings → Subscription → toggle Yearly →
modal title reads `Switch Pro to yearly billing?` and body shows `Save
15%`, `$42.50/month`, `$510.00/year`, `instead of $600.00/year`, plus
the proration line.
- [ x Max monthly subscriber → same flow → modal shows `Switch Max to
yearly billing?`, `$272.00/month`, `$3,264.00/year`, `instead of
$3,840.00/year`.
- [x] Confirm `Switch to Yearly` → Stripe receives the correct yearly
price ID and charges the prorated difference; subscription refetch
reflects yearly cadence.
  - [x] Cancel button closes the modal without changing the plan.
- [x] Yearly → monthly direction still shows the original end-of-period
copy (no charge today).
- [x] No copy implies the user gets new features / more usage / better
models from switching.
- [x] `pnpm test:unit` — billing-cards integration tests pass (existing
Pro test updated, new Max test added).

#### For configuration changes:

- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)