refactor: extract FileBrowser and shared UI primitives (Task 1.2)

- Extract FileBrowser from inline definition in repo-workspace.tsx
- Create components/features/git/FileBrowser.tsx with module CSS
- Create reusable UI primitives: LoadingState, ErrorState, StatusBadge
- Create barrel exports for components/ui/ and components/features/git/
- Replace inline loading/error patterns in dashboard, sessions, repo-workspace

Quality gates: tsc (pass), eslint (pass)
Refs: repo-restructure Task 1.2

apps/web/src/pages/sessions.tsx

@@ -19,6 +19,8 @@ import type { Session } from "../types/session";
 import type { GitRepository } from "../types/git-repository";
 import type { ToolType } from "../types/tool-type";
 import { Icon } from "../components/icon";
+import { LoadingState } from "../components/ui";
+import { ErrorState } from "../components/ui";
 
 type SessionsStatus = "loading" | "ready" | "error";
 type CreateStatus = "idle" | "creating" | "error";
@@ -271,20 +273,15 @@ export const SessionsPage = () => {
 				<h1>Sessions</h1>
 			</div>
 
-			{status === "loading" && <p className="muted">Loading sessions...</p>}
+			{status === "loading" && (
+				<LoadingState message="Loading sessions..." />
+			)}
 
 			{status === "error" && (
-				<div className="card stack">
-					<p>Failed to load sessions</p>
-					<button
-						className="secondary-button"
-						onClick={() => void loadSessions()}
-						type="button"
-					>
-						<Icon name="refresh" size="sm" />
-						Retry
-					</button>
-				</div>
+				<ErrorState
+					message="Failed to load sessions"
+					onRetry={() => void loadSessions()}
+				/>
 			)}
 
 			{status === "ready" && (