Footer Subscribe button bypasses the current Build Canada subscribe experience

## Problem

The `Subscribe` button in the Canada Spends footer links straight to Substack:

`src/components/MainLayout/Footer.tsx:76`

```tsx

Subscribe

```

This diverges from the Build Canada `Subscribe` experience as it now works. Build Canada's subscribe flow has moved on to a logged-in experience and collects additional information, notably **postal code**. Anyone converting from the Canada Spends footer skips all of that and lands in a bare Substack email capture.

## Why it matters

- **Data gap.** Subscribers acquired via Canada Spends arrive with an email address and nothing else. No postal code means these signups can't be segmented by riding/region the way the rest of the list can, and Canada Spends is a meaningful share of top-of-funnel traffic.
- **Two different front doors.** A visitor subscribing from Canada Spends gets a materially different flow from one subscribing on buildcanada.com, including whether an account gets created at all.
- **Attribution.** Unlike the header CTA (see #278, which deliberately carried `utm_source` / `utm_medium` / `utm_campaign` across when the URL changed), this footer link carries no UTM parameters at all, so footer-driven subscriptions aren't attributable to Canada Spends in the first place.

## What to decide

1. Should the footer point at the Build Canada subscribe experience instead of Substack directly, so Canada Spends signups collect postal code and hit the same logged-in flow?
2. If yes, what is the canonical URL for that flow? Worth pinning down before wiring it up: `www.buildcanada.com/subscribe` currently 404s, so the entry point is somewhere else.
3. Either way, add `utm_source=canadaspends&utm_medium=footer` so this converts measurably, matching the header convention.

## Notes

- The `Subscribe` string in the footer is also **not** wrapped in ``, unlike every other string in that component, so it does not appear in the `.po` catalogs and stays English on `/fr`. Small, but worth fixing in the same change.
- Raised alongside #282, which removes the dead `Get Involved` link from the same footer. Deliberately kept separate: #282 is a straightforward dead-link removal, this one needs a product decision first.

## Verification note

The 404 on `www.buildcanada.com/subscribe` and the Canada Spends footer code above are both confirmed. The specifics of the current Build Canada subscribe flow (logged-in experience, postal code collection) are as reported rather than independently verified here, since that UI is client-rendered and wasn't visible from the served HTML.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start with src/components/MainLayout/Footer.tsx:76 and compare the footer link with the header CTA described in #278. Confirm the canonical Build Canada subscribe entry point before changing the destination, then add the specified attribution parameters and wrap the label in Trans. Done means the footer uses the decided flow, preserves attribution, and exposes the label to the French catalog.

Written by the indexing model from the issue text.