feat(finder): add finder CTA to landing page

[GMNAPI]

Surface the Finder as the primary conversion entry point on the landing page.

Changes to web/src/app/page.tsx:

• Hero primary CTA: "Comprova les teves ajudes →" → /finder (was "Probar el asistente" → /chat)
• Nav header button: "Comprova ajudes" → /finder (was "Probar el chat")
• /chat link preserved as secondary CTA in hero

Rationale: The Finder is the conversion funnel. The chat is the deepening step after verdict.

Status: ✅ Implemented.

Effort: XS (~1h)
Deps: #29 (/finder route live)