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/Models/ReleaseNotesRequest.swift

@@ -0,0 +1,34 @@
+//
+//  ReleaseNotesRequest.swift
+//  Confab
+//
+//  Sheet-presentation payload for ReleaseNotesView
+//
+// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0
+// Copyright (C) 2026 Rune Olsen
+//
+// This file is part of Confab.
+//
+// Confab is licensed under the PolyForm Noncommercial License 1.0.0.
+// You may use, study, modify, and share it for any noncommercial
+// purpose. Commercial use — including selling Confab or any part of
+// it, standalone or bundled into another product or service —
+// requires a separate commercial license from the copyright holder.
+//
+// See the LICENSE file or
+// <https://polyformproject.org/licenses/noncommercial/1.0.0> for
+// the full license text. For commercial licensing, contact Rune
+// Olsen via <https://confab.no>.
+
+import Foundation
+
+/// Carries which version's release notes to show atomically to `.sheet(item:)` — avoids the
+/// two-sequential-@State-mutations race documented for sheets that show existing data.
+struct ReleaseNotesRequest: Identifiable {
+    let id = UUID()
+    /// Git tag form, e.g. "v2.5.0".
+    let versionTag: String
+    /// true: opened from the Help menu for the currently-installed version.
+    /// false: opened from the "update available" alert for a not-yet-installed version.
+    let isCurrentlyInstalled: Bool
+}