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/repo-workspace.tsx

@@ -1,4 +1,5 @@
 import { useCallback, useEffect, useState } from "react";
+import { Icon } from "../components/icon";
 
 import { Link, useParams, useSearchParams } from "react-router-dom";
 
@@ -155,6 +156,7 @@ export const RepoWorkspace = () => {
             onClick={() => void loadRepositories()}
             type="button"
           >
+            <Icon name="refresh" size="sm" />
             Retry
           </button>
         </div>
@@ -338,7 +340,7 @@ const FileBrowser = ({
     <div className="file-tree">
       {path && (
         <button className="tree-entry tree-up" onClick={navigateUp} type="button">
-          📁 ..
+          <Icon name="folder" size="sm" /> ..
         </button>
       )}
       {entries.map((entry) => {
@@ -350,7 +352,7 @@ const FileBrowser = ({
             onClick={() => handleEntryClick(entry)}
             type="button"
           >
-            {entry.type === "directory" ? "📁" : "📄"} {entry.name}
+            <Icon name={entry.type === "directory" ? "folder" : "file"} size="sm" /> {entry.name}
             {fileStatus && (
               <span className={`file-status-indicator ${fileStatus}`}>
                 {fileStatus === "modified" && "M"}