Add dashboard foundation with Leptos CSR

[arferreira]

Summary

• Converts repo to Cargo workspace, adds crates/dashboard/ with Leptos 0.7 CSR app
• Implements home, login, signup, and projects dashboard pages
• Adds DashboardMiddleware + rust-embed for single-binary static file serving (non-API paths serve the SPA)
• Dev workflow: trunk serve on :8080 proxies API to :3000 with hot reload

Architecture

Browser → GET /dashboard/* → DashboardMiddleware serves embedded SPA
Browser → GET /api/v1/*    → Rapina JSON API (unchanged)
Dev:      trunk serve (:8080, proxies /api to :3000)
Prod:     single reeverb binary serves both API + dashboard

Test plan

• cargo check and cargo clippy pass on both crates
• trunk build --release produces working WASM output
• Login flow works end-to-end (register via API, login via dashboard)
• Projects list loads after authentication
• All existing API tests still pass