Drive the investor dApp from the wireframe state model

[luchobonatti]

User story / Problem statement

Which screen an investor sees is decided by state, not by a route. That decision needs one place to live.

Expected outcome

A single state model selects the screen, over mock data.

Acceptance criteria

• Every screen the prototype names is reachable by setting state
• Screen precedence is implemented and tested
• The three gate functions are pure, and tested including every rejection
• An open request blocks a new one on both sides
• No contract calls anywhere in this slice

Technical notes

Keeping the gates pure means only the adapters change when the vault lands. Blocked by #83. Parent: #30.