feat: replace browser login with imported Reddit sessions

extensions/reddit-session-exporter/README.md

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+# RedditBot Session Exporter
+
+This local Chrome extension exports `reddit.com` cookies and the current Reddit Chat Matrix credential into one JSON file for the self-hosted dashboard's **Import Reddit session** control. The file is equivalent to a login credential: do not share it, commit it, or send it to anyone.
+
+## Install and use
+
+1. In Chrome, open `chrome://extensions`, enable **Developer mode**, then choose **Load unpacked** and select this folder.
+   If it was already loaded, click its **Reload** button after updating these files so Chrome starts the new service worker.
+2. Sign in to Reddit in the same Chrome profile.
+3. Open [Reddit Chat](https://www.reddit.com/chat/) and wait for it to finish loading. The extension badge changes to `OK` when it captures the Matrix credential.
+4. Click the **RedditBot Session Exporter** toolbar icon and save `redditbot-session.json`.
+5. In RedditBot Dashboard > Settings > Access, choose **Import Reddit session** and select the exported file.
+6. Delete the downloaded JSON file once the dashboard confirms that the session is authenticated.
+
+The extension observes only requests to Reddit's Matrix homeservers, verifies the captured token with Matrix `whoami`, reads Reddit cookies, and saves the combined export locally. It does not send the dump to any server; you upload it directly to your RedditBot dashboard.