feat: add extension implementation and docs

Add manifest.json, sidepanel components, and scripts.
Include project assets and documentation files.
Remove placeholder blank file.

scripts/background.js

@@ -0,0 +1,117 @@
+// Background service worker for Auto Cover Generator
+
+// 1. Native Click Behavior
+chrome.sidePanel.setPanelBehavior({ openPanelOnActionClick: true });
+
+const openTabs = new Set();
+
+// 2. Click Handler (Native behavior handles opening, we just manage content)
+// Note: chrome.action.onClicked is ignored when openPanelOnActionClick is true.
+
+// 3. Tab State Management
+// Monitor all tab changes to ensure correct panel is shown
+chrome.runtime.onInstalled.addListener(() => {
+    initializeAllTabs();
+});
+
+chrome.runtime.onStartup.addListener(() => {
+    initializeAllTabs();
+});
+
+async function initializeAllTabs() {
+    const tabs = await chrome.tabs.query({});
+    for (const tab of tabs) {
+        await checkAndSetSidePanel(tab.id);
+    }
+}
+
+// 4. Tab Activation / Isolation Logic
+chrome.tabs.onActivated.addListener(async (activeInfo) => {
+    const tabId = activeInfo.tabId;
+    await checkAndSetSidePanel(tabId);
+});
+
+chrome.tabs.onUpdated.addListener(async (tabId, changeInfo, tab) => {
+    if (changeInfo.status === 'complete' || changeInfo.url) {
+        await checkAndSetSidePanel(tabId);
+    }
+});
+
+async function checkAndSetSidePanel(tabId) {
+    try {
+        const tab = await chrome.tabs.get(tabId);
+        if (!tab) return; // Tab doesn't exist
+
+        // If we don't have permission to see the URL, it's definitely not Gemini (since we have host_permissions for Gemini).
+        // So undefined tab.url means "Restricted/Other Site".
+        const isGemini = tab.url && tab.url.startsWith('https://gemini.google.com/');
+
+        if (isGemini) {
+            // Enable and set path for Gemini
+            await chrome.sidePanel.setOptions({
+                tabId: tabId,
+                path: 'sidepanel/panel.html',
+                enabled: true
+            });
+        } else {
+            // Disable for all other sites - this CLOSES the panel if open
+            // and ensures it stays closed when returning to the tab (until clicked manually)
+            await chrome.sidePanel.setOptions({
+                tabId: tabId,
+                enabled: false
+            });
+        }
+
+    } catch (e) {
+        // Tab might be closed or we heavily failed access
+        console.log('Cannot access tab or set panel', e);
+    }
+}
+
+chrome.runtime.onInstalled.addListener(() => {
+    chrome.sidePanel.setOptions({ enabled: false });
+});
+
+chrome.tabs.onRemoved.addListener((tabId) => {
+    openTabs.delete(tabId);
+});
+
+// 4. Message Routing
+chrome.runtime.onMessage.addListener((message, sender, sendResponse) => {
+    // Forward messages to content script
+    if (message.action === 'generateCover' || message.action === 'removeObjectFromImage') {
+        chrome.tabs.query({ active: true, currentWindow: true }, (tabs) => {
+            if (tabs[0]) {
+                chrome.tabs.sendMessage(tabs[0].id, message, (response) => {
+                    sendResponse(response);
+                });
+            }
+        });
+        return true;
+    }
+
+    if (message.action === 'downloadImage') {
+        const url = message.url;
+        const timestamp = new Date().toISOString().replace(/[:.]/g, '-');
+        const filename = `gemini_gen_${timestamp}.png`;
+
+        chrome.downloads.download({
+            url: url,
+            filename: filename,
+            conflictAction: 'uniquify'
+        }, (downloadId) => {
+            if (chrome.runtime.lastError) {
+                console.error('Download failed:', chrome.runtime.lastError);
+            } else {
+                console.log('Download started, ID:', downloadId);
+            }
+        });
+        return;
+    }
+
+    if (message.action === 'generationError') {
+        chrome.runtime.sendMessage(message).catch(() => { });
+    }
+});
+
+console.log('Auto Cover Generator: Background Service Ready');