Manual validation backlog for UI, updater, and multi-monitor improvements

[shanselman]

Purpose

Consolidate the useful Repo Assist proposals that require real Windows UI, updater, global-hotkey, or multi-monitor validation before they should become code changes.

Candidates

• Updatum 1.2.1 → 1.4.0 (from [Repo Assist] chore: bump Updatum 1.2.1 → 1.4.0 #174): review release notes, build both architectures, then test discovery, download, replacement, restart, and signed release handling.
• Live tray tooltip (from [Repo Assist] improve: update system tray tooltip with live light state and brightness #179): verify On/Off and brightness updates, Windows tooltip length limits, and behavior when the tray icon is recreated.
• Color-temperature global hotkeys (from [Repo Assist] feat: add Ctrl+Shift+Left/Right hotkeys for color temperature #184): verify registration failures, conflicts with common Windows/app shortcuts, action direction, and unregistration on exit.
• Persist control-bar visibility (from [Repo Assist] improve: persist control bar visibility across restarts #185): hide controls, restart, restore controls through the tray, and verify existing settings migration.
• WM_DISPLAYCHANGE refresh (from [Repo Assist] fix: handle WM_DISPLAYCHANGE so monitor button states refresh on hotplug #187): test monitor connect/disconnect, transient topology changes, all-monitors mode, and mixed-DPI layouts.
• Kelvin approximation label (from [Repo Assist] improve: show Kelvin approximation for color temperature in Settings #190): decide whether approximate Kelvin is desirable and verify warm/cool direction and readability in Settings.
• Persist all-monitors mode (from [Repo Assist] improve: persist all-monitors mode across restarts #192): test restart, monitor removal while closed, single-monitor fallback, and settings migration.
• Theme-aware download dialog (from [Repo Assist] improve: make DownloadProgressDialog theme-aware (dark/light mode) #198): trigger a real update download in light, dark, and high-contrast modes and verify all dynamic resources resolve.

Acceptance bar

Each candidate should become its own small PR only after the listed manual validation is available. Do not merge these as one combined feature change.