CE: Per-page meta descriptions in page_shell

[ericflo]

Why

Every shell-rendered page (/signup, /login, /integrate, /setup, /dashboard, /checkout) currently emits the same generic meta description, og:description, and twitter:description. Google collapses identical descriptions and social shares look interchangeable, which hurts SERP CTR on the conversion-critical signup/login pages.

What

Adds a small private helper in crates/modelrelay-web/src/templates.rs that returns an intent-tuned description for known page titles (sign up, log in, integrate, setup, dashboard, checkout) and falls back to the existing generic line for unknown titles. page_shell_custom now uses it for the meta description, og:description, and twitter:description tags. No callsite changes — purely internal to the shell template.

Verify

After merge + deploy:

• curl -s https://modelrelay.io/signup | grep meta\ name=\"description\" → distinctive Sign Up copy
• curl -s https://modelrelay.io/login | grep meta\ name=\"description\" → distinctive Log In copy
• curl -s https://modelrelay.io/integrate | grep meta\ name=\"description\" → distinctive Integrate copy
• / and /pricing untouched (already have static unique descriptions)

Test plan

• cargo nextest run -p modelrelay-web -p modelrelay-cloud
• cargo fmt --check
• cargo clippy -- -D warnings
• 5 new unit tests covering tuned titles + default fallback + og duplication