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/instance-list.tsx

@@ -13,6 +13,7 @@ import {
 	startInstance,
 	stopInstance,
 } from "../api/sessions";
+import styles from "./features/session/InstanceList.module.css";
 
 const API_BASE_URL =
 	import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8000";
@@ -186,8 +187,8 @@ export const InstanceList = ({
 	};
 
 	return (
-		<div className="instance-list">
-			<div className="instance-list-header">
+		<div className={styles.instanceList}>
+			<div className={styles.instanceListHeader}>
 				<h3>Tool Instances</h3>
 				<button
 					className="secondary-button small"
@@ -206,30 +207,30 @@ export const InstanceList = ({
 			) : instances.length === 0 ? (
 				<p className="muted">No instances yet. Launch a tool to get started.</p>
 			) : (
-				<div className="instance-grid">
+				<div className={styles.instanceGrid}>
 					{instances.map((instance) => (
-						<div key={instance.id} className="instance-card">
-							<div className="instance-info">
-								<div className="instance-name">
+						<div key={instance.id} className={styles.instanceCard}>
+							<div className={styles.instanceInfo}>
+								<div className={styles.instanceName}>
 									{instance.display_name ||
 										instance.tool_type_name ||
 										"Unnamed Instance"}
 								</div>
-								<div className="instance-meta">
+								<div className={styles.instanceMeta}>
 									<span
-										className="status-dot"
+										className={styles.statusDot}
 										style={{ backgroundColor: getStatusColor(instance.status) }}
 									/>
 									{instance.status}
 									{isTunnelUnhealthy(instance) && (
-										<span className="error-badge" title="Tunnel unreachable">
+										<span className={styles.errorBadge} title="Tunnel unreachable">
 											<Icon name="warning" size="sm" />
 											tunnel error
 										</span>
 									)}
 								</div>
 							</div>
-							<div className="instance-actions">
+							<div className={styles.instanceActions}>
 								{instance.status === "running" &&
 									instance.url &&
 									instance.tool_type_interfaces.includes("web") && (
@@ -286,7 +287,7 @@ export const InstanceList = ({
 								{instance.status === "running" && (
 									<>
 										{stopConfirmId === instance.id ? (
-											<div className="inline-confirm">
+											<div className={styles.inlineConfirm}>
 												<span>Stop?</span>
 												<button
 													className="ghost-button small danger-text"