Share the one HTTP envelope and the one byte formatter

Both stores carried their own ~28-line request<T>, and formatBytes existed
twice – the second copy had lost the "B" branch, so a small sprite would have
read as 0 KB had its caller ever passed one under a kilobyte.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/store/HttpProjectStore.ts

@@ -1,5 +1,5 @@
 import type { Project } from '../model/Project';
-import { statusMessage } from './http';
+import { apiRequest } from './http';
 import {
   type LoadedProject,
   type ProjectStore,
@@ -8,7 +8,6 @@ import {
 } from './ProjectStore';
 import { deserializeProject, serializeProject } from './serialize';
 
-/** Routing goes through the `r` query parameter because the site's nginx config rejects PATH_INFO. */
 export class HttpProjectStore implements ProjectStore {
   constructor(private readonly endpoint: string) {}
 
@@ -46,34 +45,19 @@ export class HttpProjectStore implements ProjectStore {
     await this.request<void>('DELETE', `projects/${encodeURIComponent(id)}`);
   }
 
-  /** 409 becomes a `StaleRevisionError`, every other failure an `Error` with the server's message. */
   private async request<T>(method: string, route: string, payload?: unknown): Promise<T> {
-    const url = `${this.endpoint}?r=${encodeURIComponent(route)}`;
-    const response = await fetch(url, {
+    return apiRequest<T>(
+      this.endpoint,
       method,
-      headers: payload !== undefined ? { 'Content-Type': 'application/json' } : {},
-      body: payload !== undefined ? JSON.stringify(payload) : undefined,
-    });
-    if (response.status === 204) return undefined as T;
-    const text = await response.text();
-    let data: unknown;
-    try {
-      data = text ? JSON.parse(text) : null;
-    } catch {
-      // nginx rejects an oversized body itself and answers with an HTML page, so a failure is
-      // explained by its status rather than as "invalid JSON".
-      if (response.ok) throw new Error(`API ${method} ${route}: invalid JSON (${response.status})`);
-      data = null;
-    }
-    if (response.status === 409) {
-      const rev = (data as { revision?: number } | null)?.revision ?? -1;
-      throw new StaleRevisionError(rev);
-    }
-    if (!response.ok) {
-      const message =
-        (data as { error?: string } | null)?.error ?? statusMessage(response.status, response.statusText);
-      throw new Error(`API ${method} ${route}: ${message} (${response.status})`);
-    }
-    return data as T;
+      route,
+      {
+        headers: payload !== undefined ? { 'Content-Type': 'application/json' } : {},
+        body: payload !== undefined ? JSON.stringify(payload) : undefined,
+      },
+      (status, data) =>
+        status === 409
+          ? new StaleRevisionError((data as { revision?: number } | null)?.revision ?? -1)
+          : undefined,
+    );
   }
 }