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

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Gitty",
-  "version": "2026.8.5",
+  "version": "2026.8.6",
   "identifier": "com.gitty",
   "build": {
     "beforeDevCommand": "npm run prepare:lfs && npm run dev",