feat: project infrastructure and MV3 extension setup

TypeScript + esbuild build system, Manifest V3 with background script,
content script for iframe overlay injection, and typed message contracts
between extension contexts.

Assisted-by: Claude Code
Signed-off-by: Avinal Kumar <avinal.xlvii@gmail.com>

src/chrome.d.ts

@@ -0,0 +1,78 @@
+declare namespace chrome {
+  namespace tabs {
+    interface Tab {
+      id?: number;
+      windowId?: number;
+      title?: string;
+      url?: string;
+      favIconUrl?: string;
+      active?: boolean;
+    }
+    function query(queryInfo: Record<string, unknown>): Promise<Tab[]>;
+    function get(tabId: number): Promise<Tab>;
+    function create(createProperties: { url?: string; active?: boolean }): Promise<Tab>;
+    function update(tabId: number, updateProperties: { active?: boolean; url?: string }): Promise<Tab>;
+    function remove(tabId: number | number[]): Promise<void>;
+    function sendMessage(tabId: number, message: unknown): void;
+  }
+
+  namespace windows {
+    function update(windowId: number, updateInfo: { focused?: boolean }): Promise<void>;
+  }
+
+  namespace history {
+    interface HistoryItem {
+      id: string;
+      title?: string;
+      url?: string;
+      lastVisitTime?: number;
+    }
+    function search(query: { text: string; maxResults?: number; startTime?: number }): Promise<HistoryItem[]>;
+  }
+
+  namespace bookmarks {
+    interface BookmarkTreeNode {
+      id: string;
+      title: string;
+      url?: string;
+      children?: BookmarkTreeNode[];
+    }
+    function search(query: string): Promise<BookmarkTreeNode[]>;
+  }
+
+  namespace sessions {
+    interface Session {
+      tab?: tabs.Tab & { sessionId?: string };
+      window?: { sessionId?: string };
+    }
+    function getRecentlyClosed(filter?: { maxResults?: number }): Promise<Session[]>;
+    function restore(sessionId: string): Promise<Session>;
+  }
+
+  namespace commands {
+    const onCommand: {
+      addListener(callback: (command: string) => void): void;
+    };
+  }
+
+  namespace runtime {
+    function getURL(path: string): string;
+    const onMessage: {
+      addListener(
+        callback: (
+          message: unknown,
+          sender: { tab?: tabs.Tab; id?: string },
+          sendResponse: (response?: unknown) => void
+        ) => boolean | void
+      ): void;
+    };
+    function sendMessage(message: unknown, responseCallback?: (response: unknown) => void): void;
+  }
+
+  namespace storage {
+    const sync: {
+      get(keys: string | string[]): Promise<Record<string, unknown>>;
+      set(items: Record<string, unknown>): Promise<void>;
+    };
+  }
+}