fix(release): target stable in release-please.yml + undo the bogus 2.7.0 bump

The stable release-please workflow (release-please.yml, triggered on
push to stable) had no `target-branch`, so it defaulted to the repo
default branch (main) and computed the next version from main's stale
`.release-please-manifest.json` (2.6.1) — cutting a spurious **v2.7.0**
stable release (a version regression from 3.44.0) when #771 landed on
stable, and bumping main's package.json + manifest to 2.7.0.

- release-please.yml: add `target-branch: stable` so it releases from
  the stable branch (3.44.0 → 3.45.0), like release-please-beta.yml
  already pins `target-branch: main`.
- Restore main's version to 3.83.0-beta.0 (backend + frontend
  package.json), set `.release-please-manifest.json` to 3.44.0, and drop
  the bogus 2.7.0 CHANGELOG section.

The v2.7.0 tag/release is deleted separately; the real v3.45.0 stable is
cut by re-running release-please on the stable branch after this lands.

backend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "picpeak-backend",
-  "version": "2.7.0",
+  "version": "3.83.0-beta.0",
   "description": "Backend for PicPeak event photo sharing platform",
   "main": "server.js",
   "scripts": {