feat: role selection on wallet connect with adaptive navigation and CTAs

[johneliud]

Problem

The current navigation exposes both /client and /freelancer routes simultaneously, which is a known marketplace UX anti-pattern. Users have no clear entry point and the dual-nav is confusing.

Solution

Implement role selection on wallet connect (Option 1 from UX research):

• After a user connects their Freighter wallet for the first time, show a role selector: "I want to hire" (Client) or "I want to work" (Freelancer)
• Persist the choice in localStorage so returning users are taken straight to their dashboard
• Navbar collapses to show only role-relevant links
• Landing page CTA buttons adapt based on stored role
• Provide a role switcher so users can change later

Acceptance Criteria

• Role selector appears after wallet connect if no role is stored
• Role is persisted in localStorage under milestack_role
• Navbar shows only links relevant to the selected role
• Landing page hero CTAs adapt per role (or show dual CTAs when no role)
• Role can be changed via a switcher in the navbar or dashboard
• No regression on existing client/freelancer page functionality