chore: move production output to prod

[vondutchi]

Addresses #22 as a focused backup implementation. The issue text mentions an assignee, but GitHub shows no current assignee and I could not find a matching PR, so I kept this narrow and easy to close if it is no longer wanted.

Changes:

• Centralizes the Rollup production output folder as prod and writes extension bundles/assets/manifest there.
• Renames bundle visualizer outputs from stats-* to kleo-*.
• Removes stale tracked stats-*.html reports and ignores generated prod/ plus kleo-*.html reports.
• Adds npm run test:rollup-config to guard the Rollup output paths and visualizer naming.
• Documents the production build behavior in README.md.

Verification:

• npm run test:rollup-config
• npm run build
• git diff --check

Build notes:

• npm ci failed because the existing lockfile is out of sync, so I installed dependencies with npm install --legacy-peer-deps before running the build.
• The build completes with existing Rollup/Browserslist warnings: unused rollup-plugin-url, outdated caniuse-lite, aes-js this rewrite warnings, and existing viem circular dependencies.

If this is accepted for the 50 USDC bounty, I can provide payout details privately.