Fix flash of unstyled content (FOUC) on page load

[ankit]

Summary

Custom styles could briefly fail to apply on page load, causing a flash of unstyled content (FOUC) before Stylebot's CSS kicked in.

Root cause: the inject-css content script runs at document_start, but it still had to chrome.runtime.sendMessage the background service worker and wait for it to read chrome.storage.local before the <style> tag could be inserted. In Manifest V3, the service worker can be asleep between uses, and waking it up added enough latency to produce a visible flash — worse, the script only detected a failed lookup and retried every 300ms, up to 10 times, if the service worker wasn't ready in time.

Fix: the content script now reads chrome.storage.local directly and applies the existing URL-matching logic itself, entirely skipping the message round trip to the background page. chrome.storage is handled by the browser process, not the extension's service worker, so styles apply immediately regardless of whether the service worker is awake. The old retry-polling fallback is no longer needed and has been removed.

Changes

• src/inject-css/index.ts: read chrome.storage.local directly instead of messaging the background page; drop the retry-polling fallback.
• src/styles/ (new): extracted the pure getStylesForPage URL-matching logic and BackgroundPageUtils out of src/background/ into a shared package (@stylebot/styles) so both the background page and the content script can use it without the content script depending on background-only code.
• src/background/listeners.ts: badge/icon updates now happen in the existing chrome.tabs.onUpdated listener, since they used to piggyback on the message the content script no longer sends.
• Removed the GetStylesForIframe message type/handler, which was only ever sent by the old content script and is now dead code.
• webpack.config.js: added a resolve alias for the new @stylebot/styles package.

Test plan

• npx jest — all 76 tests pass
• npx eslint --resolve-plugins-relative-to . src — no lint errors on touched files
• yarn build compiles successfully
• Manually loaded the built extension in Chrome and confirmed custom CSS applies immediately on page load/reload with no visible flash