fix(frontend): use oven/bun base image to fix arm64 build failure

Replaced `node:22-alpine` + `npm install -g bun@1.2.22` with
`oven/bun:1.2.22-alpine` as the builder stage base image.

The npm-based bun install was failing on linux/arm64 with:
  "Failed to find package @oven/bun-linux-aarch64"

The official oven/bun Docker image supports both linux/amd64 and
linux/arm64 natively, eliminating the need to install bun via npm.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/Dockerfile

@@ -1,5 +1,5 @@
-# Use this image as the platform to build the app
-FROM node:22-alpine AS builder
+# Use the official Bun image as the build platform (supports linux/amd64 and linux/arm64 natively)
+FROM oven/bun:1.2.22-alpine AS builder
 
 # Metadata labels for the AdventureLog image
 LABEL maintainer="Sean Morley" \
@@ -21,9 +21,6 @@ WORKDIR /app
 # Upgrade zlib to include Alpine security fixes
 RUN apk upgrade --no-cache zlib
 
-# Install bun toolchain
-RUN npm install -g bun@1.2.22
-
 # Copy package files first for better Docker layer caching
 COPY package.json bun.lock* ./