Show release notes in-app instead of opening the web releases page

New "Read Release Notes" entries in the Help menu (current installed
version) and the "Check for Updates" alert (the new, not-yet-installed
version) render a release's markdown notes in a Confab modal.

- UpdateCheckService.fetchReleaseNotes(forTag:) fetches a release's
  title + body from Gitea's public releases-by-tag API, caching the
  result by version tag in the settings table (a published release's
  notes don't change, so no need to refetch on every view).
- ReleaseNotesView reuses the existing MarkdownContentView renderer;
  shows a friendly "not available yet" state for versions with no
  published Gitea release (e.g. a dev build ahead of the last release).
- ReleaseNotesRequest carries which version to show atomically via
  .sheet(item:), per this project's established sheet-timing pattern.
- Removed the redundant "Release Page" button from the update alert
  now that notes show in-app; added an explicit .keyboardShortcut
  (.cancelAction) to its cancel button so Escape actually closes it —
  role: .cancel alone didn't do it, since NSAlert only auto-binds
  Escape to a button literally titled "Cancel".

oAI/oAIApp.swift

@@ -209,6 +209,10 @@ struct oAIApp: App {
                 Button("Confab Help") { openHelp() }
                     .keyboardShortcut("?", modifiers: .command)
                 Divider()
+                Button("Read Release Notes") {
+                    let current = Bundle.main.infoDictionary?["CFBundleShortVersionString"] as? String ?? ""
+                    chatViewModel.releaseNotesRequest = ReleaseNotesRequest(versionTag: "v\(current)", isCurrentlyInstalled: true)
+                }
                 Button(UpdateCheckService.shared.isCheckingManually ? "Checking…" : "Check for Updates…") {
                     UpdateCheckService.shared.checkForUpdatesManually()
                 }