Update: UpdaterDialog QML — states, changelog viewer, channel picker

[fernandotonon]

Parent epic: #439
Depends on: #441, #443.

Goal

A polished QML dialog covering every updater state, hosting the changelog and the per-flavor guidance.

Scope

• New qml/UpdaterDialog.qml:
  • Single root Dialog (or Popup) styled to match PropertiesPanel.qml / MaterialEditorWindow.qml.
  • States bound to UpdaterController.state: Idle, Checking, UpToDate, UpdateAvailable, Downloading, Verifying, ReadyToInstall, Error, PackageManaged.
  • UpdateAvailable state: title + version + published date + Markdown changelog (use Qt.labs.qmlmodels + a TextArea with textFormat: TextEdit.MarkdownText).
  • Downloading: progress bar bound to UpdaterController.progress, cancel button.
  • PackageManaged: copy-pasteable command with copy button (per #Update: Install-flavor detection + package-manager passthrough UX #443).
  • "Remind me later" closes; "Skip this version" stores tag in QSettings so it doesn't re-prompt until a newer version appears.
• qml/UpdaterSettings.qml panel embedded in the existing settings dialog (wherever the LLM settings live):
  • Channel picker (Stable/Beta/Nightly).
  • "Check on startup" toggle.
  • "Auto-download in background" toggle (default OFF).
  • "Last checked" timestamp + manual Check now button.
• Help menu Verify Update renamed to Check for Updates… and routed to UpdaterController::checkForUpdates().

Acceptance criteria

• All 9 states render correctly under qmllint and a snapshot/golden test.
• Changelog Markdown renders inline.
• Skip-this-version persists across restarts.
• Settings persist via QSettings.
• Sentry breadcrumb updater.dialog.* for state transitions.

Effort

~1 week.

[fernandotonon]

Shipped in 3.6.0 (auto-updater MVP).