chore(release): bump to 2026.8.6 and update changelog

Bump version to 2026.8.6 across packaging, app config, and docs.
The change aligns binaries, config files, and changelog with the new release.
A new 2026.8.6 entry is added to the changelog and the UI surfaces release notes.

- Aligns version across packaging, app config, and UI
- Adds 2026.8.6 changelog entry and release notes in UI
- Prepares release by updating docs and build metadata

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gitty",
-  "version": "2026.8.5",
+  "version": "2026.8.6",
   "private": true,
   "type": "module",
   "scripts": {