refactor: consolidate loading/error states and extract instance actions hook

Frontend:
- Create reusable DataStates components (LoadingState, ErrorState, EmptyState)
- Refactor 12 pages to use shared state components instead of inline JSX
- Extract useInstanceActions hook to eliminate session action duplication
- Update dashboard and sessions pages to use shared hook

OpenSpec:
- Archive completed mobile-app-usability change (44/44 tasks)
- Archive completed add-config-profiles change (15/15 tasks)

Quality: TypeScript check passes, production build succeeds

apps/web/src/pages/ssh-keys.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { useNavigate } from "react-router-dom";
 import { createSSHKey, deleteSSHKey, listSSHKeys, signPayload, verifySignature, type SSHKey } from "../api/ssh_keys";
+import { EmptyState, ErrorState, LoadingState } from "../components/data-states";
 import { Icon } from "../components/icon";
 import { useAsyncData } from "../hooks/use-async-data";
 
@@ -87,7 +88,7 @@ export const SSHKeysPage = () => {
     }
   }
 
-  if (status === "loading") return <div>Loading...</div>;
+  if (status === "loading") return <LoadingState message="Loading SSH keys..." />;
 
   return (
     <section className="stack">
@@ -130,19 +131,11 @@ export const SSHKeysPage = () => {
         </button>
       </form>
 
-      {status === "error" && (
-        <div className="card stack">
-          <p>Failed to load SSH keys</p>
-          <button className="secondary-button" onClick={() => loadKeys()} type="button">
-            <Icon name="refresh" size="sm" />
-            Retry
-          </button>
-        </div>
-      )}
+      {status === "error" && <ErrorState message="Failed to load SSH keys" onRetry={loadKeys} />}
 
       <div className="keys-list">
         {safeKeys.length === 0 ? (
-          <p className="muted">No SSH keys yet. Generate one above.</p>
+          <EmptyState message="No SSH keys yet. Generate one above." />
         ) : (
           safeKeys.map((key) => (
             <div key={key.id} className="key-card">