* fix(web): exclude PostHog proxy path from i18n middleware
The next-intl middleware added in cf75da8f intercepts /cmuxterm/* requests
(the PostHog reverse proxy), causing analytics to break since March 12.
Add cmuxterm to the negative lookahead so proxy requests bypass i18n.
* refactor(web): rename middleware.ts to proxy.ts for Next.js 16
Next.js 16 renamed middleware.ts to proxy.ts. Migrate to the new
convention since we're on Next.js 16.1.6.
* feat(web): migrate PostHog to managed reverse proxy at r.cmux.dev
Replace the Next.js rewrites-based proxy (/cmuxterm -> us.i.posthog.com)
with PostHog's managed reverse proxy at r.cmux.dev. This removes the
rewrites from next.config.ts entirely and eliminates the proxy.ts
matcher conflict that caused the analytics regression.
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Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>
