fix: replace frontend with Rust OPAQUE API + Flutter keypad UI

- Full OPAQUE auth flow via WASM client SDK (client-wasm crate)
- New user: Key Register → Key Login → Code Register (icon selection) → done
- Existing user: Key Login → get login-data → icon keypad → Code Login → done
- Icon-based keypad matching Flutter design:
  - 2 cols portrait, 3 cols landscape
  - Key tiles with 3-col sub-grid of icons
  - Navy border press feedback
  - Dot display with backspace + submit
- SVGs rendered as-is (no color manipulation)
- SusiPage with Login/Signup tabs
- LoginKeypadPage and SignupKeypadPage for code flows
- Secret key display/copy on signup
- Unit tests for Keypad component
- WASM pkg bundled locally (no external dep)

src/vite-env.d.ts

@@ -0,0 +1,9 @@
+/// <reference types="vite/client" />
+
+interface ImportMetaEnv {
+  readonly VITE_API_BASE_URL: string
+}
+
+interface ImportMeta {
+  readonly env: ImportMetaEnv
+}