refactor(ext/shared): typed StateHost + sweep as-any casts (Plan C Phase 1)

Replaces the previously any-typed StateHost contract with a typed interface.
Adds double-registration guard and __resetHostForTests for vitest.
sendMessage wrapper is currently a pass-through; Phase 4 will fill its body
with the vault_locked intercept lifted from vault.ts.

Widens PopupState/View on shared/popup-state.ts to cover vault-tab-only
views (history, backup, import) and vault-tab-only fields (unlocked,
drawerOpen, typePanelOpen) so VaultState satisfies StateHost.getState()
without a cast. The popup surface ignores the new optional fields.

Drops the `any` annotations on vault.ts's registerHost callbacks now that
the typed StateHost contract infers them from PopupState.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

extension/src/vault/vault.ts

@@ -229,11 +229,11 @@ const state: VaultState = {
 
 registerHost({
   getState: () => state,
-  setState: (partial: any) => {
+  setState: (partial) => {
     Object.assign(state, partial);
     renderPane();
   },
-  navigate: (view: string, extras?: any) => {
+  navigate: (view, extras) => {
     Object.assign(state, { view, error: null, loading: false, ...extras });
     setHash(view as VaultView);
     applyShellViewClass();