Add 1-day local cache for style files fetched from GitHub

[Copilot]

CSS files were fetched from GitHub on every page load and every browser startup/navigation with no caching. This adds a 1-day TTL cache using chrome.storage.local.

Key changes

• getCachedOrFetch(url) — new helper in both service_worker.js and content.js that checks chrome.storage.local before hitting the network. Cache entries are stored as cssCache_<url> → { css, cachedAt } and expire after 24h.
• getThemes() caching — themes.json metadata is now cached in local storage (themesJson + themesJsonCachedAt), re-fetched only after 1 day.
• chrome.storage.sync → chrome.storage.local for themes — sync storage has a 100KB total quota, which is insufficient for CSS content. Local storage (10MB) is used instead. activeThemes user preference remains in sync storage.
• Deferred DOM injection — activateTheme() now uses Promise.all() to wait for all CSS files before appending the <style> element, preventing partial styling.
• Fixed CSS concatenation bug — service worker was overwriting theme.css on each file instead of appending; also now saves to storage only after all fetches complete.

function getCachedOrFetch(url) {
  return new Promise((resolve, reject) => {
    const cacheKey = 'cssCache_' + url;
    chrome.storage.local.get([cacheKey], (result) => {
      const cached = result[cacheKey];
      if (cached && cached.cachedAt && (Date.now() - cached.cachedAt) < CACHE_DURATION) {
        resolve(cached.css);
      } else {
        fetch(url)
          .then((res) => res.text())
          .then((css) => {
            chrome.storage.local.set({ [cacheKey]: { css, cachedAt: Date.now() } });
            resolve(css);
          })
          .catch(reject);
      }
    });
  });
}

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add CDN or caching to style files</issue_title>
<issue_description></issue_description>

<agent_instructions>Download files from github and store at each user locally. Instead of fetching everytime. Store it for 1 day currently. Do not ship files inside browser extensions.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Add CDN or caching to style files #11

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