Add update notifications and rolling cluster upgrades for 1.1.0

[drudge]

Users currently have to check manually for new releases and update each cluster node separately. This adds release discovery after sign-in and a reviewed, sequential cluster upgrade flow for 1.1.0.

• Show a persistent update notification from any page, including the Check for Updates quick action. Share the release-notes dialog with About and Cluster, keep installation/restart progress in the notification, and confirm successful node updates after restart.
• Offer node or cluster updates when permissions and every node's capabilities allow it. Remember the selected scope per user in the browser; automatic checks can be disabled in Settings → General.
• Pin a rolling upgrade to the reviewed release, update replicas one at a time, wait for health and synchronization, and update the primary last. Keep progress visible through restarts, with numbered waiting steps, active spinners, and completion indicators.
• Persist release notes across restarts, link versions to GitHub, and require curated changelog sections when publishing instead of generated commit lists.
• Add a disposable three-node demo with a local release feed and continuous DNS probes. Document the final UI and add curated 1.1.0 release notes.

Rolling upgrades require writable binaries and managed restarts on every node. Failures, timeouts, or an operator stop prevent further restarts. DNS clients must use multiple servers to retain service while a node restarts.

Validation

• go tool mage verify — generated files, full Go test suite, and static analysis pass.
• go tool mage race — full race suite passes.
• Module verification, module tidiness, formatting, and release-note extraction for 1.1.0 pass.
• Manually verified desktop/mobile notifications, remembered update scope, installation and restart flow, and complete three-node rollouts with DNS remaining available on other nodes.
• Browser regression fixtures compile and scripts pass syntax checks; the automated browser suite runs in CI.