Refactor build pipeline for performance and dev output reliability

Replace JS/CSS minifiers with esbuild to reduce build times while
preserving outputs.

Enable `thread-loader` by default in dev and production. Choose workers
dynamically (based on CPU cores) and allow overrides via environment
variables. Keep filesystem cache enabled and make cache compression
configurable, defaulting to uncompressed for faster warm builds on
CPU-bound machines. Add BUILD_PARALLEL toggle (default on) to switch
between parallel and sequential production variant builds. Ensure
watch-once dev builds exit cleanly by awaiting async output work.

Adopt `sass-embedded` for SASS processing. In development, use
`style-loader` to speed up CSS/SCSS compilation while keeping production
outputs unchanged. Maintain CSP-safe source maps for extension contexts;
copy external maps in dev to avoid 404s; suppress CSS 404 noise in dev
outputs by skipping only missing CSS.

Additionally, dependency caching has been added to the GitHub Actions
workflow to accelerate CI/CD runs.

Results on this DO VPS (2 cores, ~4 GiB RAM):
- Production first run (cold): ~44s (baseline ~105s)
- Production second run (warm): ~19s (baseline ~39s)
- Development first run: ~31s; second run: ~29s

Times vary by environment; numbers above are for this machine.

Author: PeterDaveHello

.github/copilot-instructions.md

@@ -17,6 +17,25 @@ Always reference these instructions first and fall back to search or bash comman
 - Lint code: `npm run lint` -- uses ESLint.
 - Safari build: `npm run build:safari` (see Platform-Specific Instructions for details)
 
+### Build Performance Options
+
+- BUILD_PARALLEL: Toggle parallel build of production variants
+  - Default: on (parallel). Set to `0` to run sequentially (lower CPU/IO spikes on low-core machines)
+- BUILD_THREAD / BUILD_THREAD_WORKERS: Control Babel parallelism via thread-loader
+  - Default: threads enabled in dev/prod; workers = CPU cores
+  - Set `BUILD_THREAD=0` to disable; set `BUILD_THREAD_WORKERS=<n>` to override worker count
+- BUILD_CACHE_COMPRESSION: Webpack filesystem cache compression
+  - Default: `0` (no compression) for faster warm builds on CPU‑bound SSD machines
+  - Options: `0|false|none`, `gzip` (or `brotli` if explicitly desired)
+  - Affects only `.cache/webpack` size/speed; does not change final artifacts
+- BUILD_WATCH_ONCE (dev): When set, `npm run dev` runs a single build and exits (useful for timing)
+- BUILD_POOL_TIMEOUT: Override thread-loader production pool timeout (ms)
+  - Default: `2000`. Increase if workers recycle too aggressively on slow machines/CI
+- Source maps (dev): Dev builds emit external `.map` files next to JS bundles for CSP‑safe debugging; production builds disable source maps
+- Symlinks: Webpack uses `resolve.symlinks: false` to improve performance and ensure consistent module identity; if you rely on `npm link`/pnpm workspaces, temporarily enable symlink resolution while developing linked packages
+
+Performance defaults: esbuild handles JS/CSS minification. In development, CSS is injected via style-loader; in production, CSS is extracted via MiniCssExtractPlugin. Thread-loader is enabled by default in both dev and prod.
+
 ### Build Output Structure
 
 Production build creates multiple variants in `build/` directory:

.github/workflows/pre-release-build.yml

@@ -24,7 +24,25 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+      - name: Detect Node major version
+        run: echo "NODE_MAJOR=$(node -p 'process.versions.node.split(".")[0]')" >> $GITHUB_ENV
+      - name: Cache npm cache
+        uses: actions/cache@v4
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node-
       - run: npm ci
+      - name: Cache Webpack filesystem cache
+        uses: actions/cache@v4
+        with:
+          path: |
+            .cache/webpack
+            node_modules/.cache/webpack
+          key: ${{ runner.os }}-node${{ env.NODE_MAJOR }}-webpack-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node${{ env.NODE_MAJOR }}-webpack-
       - run: npm run build
 
       - uses: josStorer/get-current-time@v2