fix: make the OS title bar follow the active theme

[laurentiu021]

Closes #1622.

The problem

ApplyDarkTitleBar passed a hardcoded int value = 1 to DWMWA_USE_IMMERSIVE_DARK_MODE and ran exactly once from OnSourceInitialized — no second call site, no theme subscription. So the title bar stayed dark for the life of the process.

On any of the six light presets the app was a near-white window wearing a black strip: the one piece of chrome the user could not theme away, and the last surface still pinned to dark after every brush, chart paint and control template had already been migrated to invert per preset.

The fix

ApplyTitleBarTheme(hwnd, dark) passing dark ? 1 : 0 (0 is the documented default), called with ThemeService.Instance.CurrentTheme.IsDark, plus a ThemeChanged subscription so a switch re-applies immediately rather than waiting for a restart.

Unsubscribed in OnClosed: ThemeService is a singleton that outlives the window, so a surviving handler would keep this instance alive and fire against a dead handle.

Verification

I checked the signal the fix depends on rather than trusting it. IsDark classifies all 12 presets correctly against the same R+G+B < 384 threshold the record applies to custom themes — and the split is 6 dark / 6 light, so the old hardcoded 1 was giving six presets the wrong title bar:

deep-ocean       bg=#050D1A sum= 44  IsDark=True  -> DWM 1
lavender         bg=#FAF5FF sum=750  IsDark=False -> DWM 0
midnight-indigo  bg=#070A0F sum= 32  IsDark=True  -> DWM 1
mint-fresh       bg=#F0FDF4 sum=737  IsDark=False -> DWM 0
neon-rose        bg=#120508 sum= 31  IsDark=True  -> DWM 1
sky-breeze       bg=#F8FAFC sum=750  IsDark=False -> DWM 0
soft-blossom     bg=#FDF2F8 sum=743  IsDark=False -> DWM 0
violet-night     bg=#0A0515 sum= 36  IsDark=True  -> DWM 1
warm-ember       bg=#0F0A04 sum= 29  IsDark=True  -> DWM 1
warm-sand        bg=#FFFBEB sum=741  IsDark=False -> DWM 0

15/15 on that harness. Pinned in the suite as a theory over every preset, plus a guard that both dark and light presets ship — if every preset were dark the theory would be silently vacuous and pass while testing nothing.

All four projects rebuild --no-incremental with 0 errors, 0 warnings.

Scope

DwmSetWindowAttribute needs a real window, so the on-screen result is deferred to the secondary workstation — what's checkable here is the decision feeding it. The return value stays discarded: the attribute is unsupported before Windows 10 1809, and failing to tint a title bar must never take the window down.

MainWindow is the only Window in the project, so this is the only affected frame.