feat: implement universal icon system with Phosphor Icons

- Install @phosphor-icons/react package
- Create centralized Icon component with size/weight/color variants
- Create icon registry with 34 icons across 5 categories
- Replace all raw Unicode symbols with proper icon components
- Add icons to navigation, buttons, status indicators, git operations
- Add icon CSS with consistent sizing and spacing
- Fix type definitions for Phosphor icon compatibility

Quality gates: typecheck ✓, lint ✓, build ✓ (375KB bundle)

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

@@ -1,5 +1,6 @@
 import { useEffect, useState } from "react";
 import { createSSHKey, deleteSSHKey, listSSHKeys, type SSHKey } from "../api/ssh_keys";
+import { Icon } from "../components/icon";
 
 export const SSHKeysPage = () => {
   const [keys, setKeys] = useState<SSHKey[]>([]);
@@ -77,7 +78,17 @@ export const SSHKeysPage = () => {
           />
         </div>
         <button type="submit" className="primary-button" disabled={generating}>
-          {generating ? "Generating..." : "Generate SSH Key"}
+          {generating ? (
+            <>
+              <Icon name="loading" size="sm" />
+              Generating...
+            </>
+          ) : (
+            <>
+              <Icon name="add" size="sm" />
+              Generate SSH Key
+            </>
+          )}
         </button>
       </form>
 
@@ -93,6 +104,7 @@ export const SSHKeysPage = () => {
                   onClick={() => handleDelete(key.id)}
                   className="danger-button"
                 >
+                  <Icon name="delete" size="sm" />
                   Delete
                 </button>
               </div>
@@ -107,6 +119,7 @@ export const SSHKeysPage = () => {
                   onClick={() => copyToClipboard(key.public_key)}
                   className="secondary-button"
                 >
+                  <Icon name="copy" size="sm" />
                   Copy Full Key
                 </button>
               </div>