phase 6: web storage layer (KeyStore, Cache, session)

KeyStore + Cache TS interfaces with WebCrypto non-extractable Ed25519
keys persisted via IndexedDB (idb), plus thin api/session.ts that
loads or creates the device session at app startup. Vitest unit
tests under fake-indexeddb cover both adapters; Playwright e2e
verifies the keypair survives reload and produces signatures still
verifiable under the persisted public key (gateway round-trip moves
to Phase 7's existing acceptance bullet).

Browser baseline: WebCrypto Ed25519 — Chrome >=137, Firefox >=130,
Safari >=17.4. No JS fallback; ui/docs/storage.md documents the
matrix and the WebKit non-determinism quirk.

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

ui/frontend/package.json

@@ -11,6 +11,9 @@
     "test": "vitest run",
     "test:e2e": "playwright test"
   },
+  "dependencies": {
+    "idb": "^8.0.3"
+  },
   "devDependencies": {
     "@bufbuild/protobuf": "^2.12.0",
     "@bufbuild/protoc-gen-es": "^2.12.0",
@@ -23,6 +26,7 @@
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/svelte": "^5.2.0",
     "@types/node": "^22.0.0",
+    "fake-indexeddb": "^6.2.5",
     "jsdom": "^25.0.0",
     "playwright": "^1.59.1",
     "svelte": "^5.0.0",