feat: add customizable business branding (name, logo, colors)

Add admin settings for business branding with name, logo upload, and
color scheme via CSS custom properties. Includes database migration,
API endpoints, admin settings page, and dynamic branding in both
admin nav and customer portal.

Closes #61

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/routes/settings.ts

@@ -0,0 +1,59 @@
+import { Hono } from "hono";
+import { zValidator } from "@hono/zod-validator";
+import { z } from "zod";
+import { eq, getDb, businessSettings } from "@groombook/db";
+
+export const settingsRouter = new Hono();
+
+// GET /api/admin/settings — return current business settings
+settingsRouter.get("/", async (c) => {
+  const db = getDb();
+  const [row] = await db.select().from(businessSettings).limit(1);
+  if (!row) {
+    // Auto-create default settings if none exist
+    const [created] = await db.insert(businessSettings).values({}).returning();
+    return c.json(created);
+  }
+  return c.json(row);
+});
+
+const hexColorRegex = /^#[0-9a-fA-F]{6}$/;
+
+const updateSettingsSchema = z.object({
+  businessName: z.string().min(1).max(200).optional(),
+  primaryColor: z.string().regex(hexColorRegex, "Must be a hex color like #4f8a6f").optional(),
+  accentColor: z.string().regex(hexColorRegex, "Must be a hex color like #8b7355").optional(),
+  logoBase64: z.string().max(700_000).nullable().optional(), // ~512KB base64
+  logoMimeType: z
+    .enum(["image/png", "image/svg+xml", "image/jpeg", "image/webp"])
+    .nullable()
+    .optional(),
+});
+
+// PATCH /api/admin/settings — update business settings
+settingsRouter.patch(
+  "/",
+  zValidator("json", updateSettingsSchema),
+  async (c) => {
+    const db = getDb();
+    const body = c.req.valid("json");
+
+    // Get or create the settings row
+    const rows = await db.select().from(businessSettings).limit(1);
+    let settingsId: string;
+    if (rows[0]) {
+      settingsId = rows[0].id;
+    } else {
+      const inserted = await db.insert(businessSettings).values({}).returning();
+      settingsId = inserted[0]!.id;
+    }
+
+    const [updated] = await db
+      .update(businessSettings)
+      .set({ ...body, updatedAt: new Date() })
+      .where(eq(businessSettings.id, settingsId))
+      .returning();
+
+    return c.json(updated);
+  }
+);