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/pnpm-lock.yaml

@@ -7,6 +7,10 @@ settings:
 importers:
 
   frontend:
+    dependencies:
+      idb:
+        specifier: ^8.0.3
+        version: 8.0.3
     devDependencies:
       '@bufbuild/protobuf':
         specifier: ^2.12.0
@@ -41,6 +45,9 @@ importers:
       '@types/node':
         specifier: ^22.0.0
         version: 22.19.17
+      fake-indexeddb:
+        specifier: ^6.2.5
+        version: 6.2.5
       jsdom:
         specifier: ^25.0.0
         version: 25.0.1
@@ -557,6 +564,10 @@ packages:
     resolution: {integrity: sha512-knvyeauYhqjOYvQ66MznSMs83wmHrCycNEN6Ao+2AeYEfxUIkuiVxdEa1qlGEPK+We3n0THiDciYSsCcgW/DoA==}
     engines: {node: '>=12.0.0'}
 
+  fake-indexeddb@6.2.5:
+    resolution: {integrity: sha512-CGnyrvbhPlWYMngksqrSSUT1BAVP49dZocrHuK0SvtR0D5TMs5wP0o3j7jexDJW01KSadjBp1M/71o/KR3nD1w==}
+    engines: {node: '>=18'}
+
   fdir@6.5.0:
     resolution: {integrity: sha512-tIbYtZbucOs0BRGqPJkshJUYdL+SDH7dVM8gjy+ERp3WAUjLEFJE+02kanyHtwjWOnwrKYBiwAmM0p4kLJAnXg==}
     engines: {node: '>=12.0.0'}
@@ -623,6 +634,9 @@ packages:
     resolution: {integrity: sha512-4fCk79wshMdzMp2rH06qWrJE4iolqLhCUH+OiuIgU++RB0+94NlDL81atO7GX55uUKueo0txHNtvEyI6D7WdMw==}
     engines: {node: '>=0.10.0'}
 
+  idb@8.0.3:
+    resolution: {integrity: sha512-LtwtVyVYO5BqRvcsKuB2iUMnHwPVByPCXFXOpuU96IZPPoPN6xjOGxZQ74pgSVVLQWtUOYgyeL4GE98BY5D3wg==}
+
   indent-string@4.0.0:
     resolution: {integrity: sha512-EdDDZu4A2OyIK7Lr/2zG+w5jmbuk1DVBnEwREQvBzspBJkCEbRa8GxU1lghYcaGJCnRWibjDXlq779X1/y5xwg==}
     engines: {node: '>=8'}
@@ -1495,6 +1509,8 @@ snapshots:
 
   expect-type@1.3.0: {}
 
+  fake-indexeddb@6.2.5: {}
+
   fdir@6.5.0(picomatch@4.0.4):
     optionalDependencies:
       picomatch: 4.0.4
@@ -1567,6 +1583,8 @@ snapshots:
     dependencies:
       safer-buffer: 2.1.2
 
+  idb@8.0.3: {}
+
   indent-string@4.0.0: {}
 
   is-potential-custom-element-name@1.0.1: {}