refactor: extract CSS modules for session/settings and delete styles.css (Task 2.3)

- Create InstanceList.module.css, AppShell.module.css, SettingsTabLayout.module.css
- Create CommitPanel.module.css, FileViewer.module.css
- Create page CSS files: sessions, repo-workspace, dashboard, projects,
  git-history, ssh-keys, settings
- Update components to import and use CSS modules
- Delete monolithic styles.css (2,255 lines)
- Update main.tsx to import page CSS and new modules

Quality gates: tsc (pass), eslint (pass), build (pass)
Refs: repo-restructure Task 2.3

apps/web/src/components/commit-panel.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 
 import { commitChanges } from "../api/git_repositories";
+import styles from "./features/git/CommitPanel.module.css";
 
 interface CommitPanelProps {
   projectId: string;
@@ -49,49 +50,49 @@ export const CommitPanel = ({
   if (!hasChanges) return null;
 
   return (
-    <div className="commit-panel">
+    <div className={styles.commitPanel}>
       <h4>Changes</h4>
       
-      <div className="file-list">
+      <div className={styles.fileList}>
         {modified.map((file) => (
-          <div key={file} className="file-item modified">
-            <span className="file-status">M</span>
-            <span className="file-name">{file}</span>
+          <div key={file} className={`${styles.fileItem} modified`}>
+            <span className={styles.fileStatus}>M</span>
+            <span>{file}</span>
           </div>
         ))}
         {added.map((file) => (
-          <div key={file} className="file-item added">
-            <span className="file-status">A</span>
-            <span className="file-name">{file}</span>
+          <div key={file} className={`${styles.fileItem} added`}>
+            <span className={styles.fileStatus}>A</span>
+            <span>{file}</span>
           </div>
         ))}
         {deleted.map((file) => (
-          <div key={file} className="file-item deleted">
-            <span className="file-status">D</span>
-            <span className="file-name">{file}</span>
+          <div key={file} className={`${styles.fileItem} deleted`}>
+            <span className={styles.fileStatus}>D</span>
+            <span>{file}</span>
           </div>
         ))}
         {untracked.map((file) => (
-          <div key={file} className="file-item untracked">
-            <span className="file-status">?</span>
-            <span className="file-name">{file}</span>
+          <div key={file} className={`${styles.fileItem} untracked`}>
+            <span className={styles.fileStatus}>?</span>
+            <span>{file}</span>
           </div>
         ))}
       </div>
 
-      <div className="commit-form">
+      <div className={styles.commitForm}>
         <textarea
           placeholder="Commit message"
           value={message}
           onChange={(e) => setMessage(e.target.value)}
           rows={2}
-          className="commit-message-input"
+          className={styles.commitMessageInput}
         />
-        {error && <div className="commit-error">{error}</div>}
+        {error && <div className={styles.commitError}>{error}</div>}
         <button
           onClick={handleCommit}
           disabled={loading || !message.trim()}
-          className="commit-button"
+          className={styles.commitButton}
           type="button"
         >
           {loading ? "Committing..." : "Commit"}