refactor: split License out of MainViewModel — Commit 2/12 (tag refactor-commit-2)

Pure member movement into MainViewModel.License.cs (zero behavior change):
Polar license state (LicenseKey/LicenseMessage/IsLicenseValidating/
IsLicenseEntryOpen/IsHubVisible/IsPremium + OfflineGracePeriod),
ValidateLicenseAtStartupAsync, ActivateLicenseAsync. docs in same commit:
ViewModels/index.md split table (License ✔).

ViewModels/index.md

@@ -27,12 +27,15 @@ Progress (Commit N/12 · tag):
 - **Commit 1/12 · `refactor-commit-1`: Account landed** → `MainViewModel.Account.cs`
   (session restore `LoadSavedSessionAsync`, sign-in/out/change account,
   `IsConnected`/`AccountAvatarUrl`/`AccountDisplayName`/`AccountStatusLightToolTip`).
+- **Commit 2/12 · `refactor-commit-2`: License landed** → `MainViewModel.License.cs`
+  (Polar license state: `LicenseKey`/`LicenseMessage`/`IsLicenseValidating`/
+  `IsLicenseEntryOpen`/`IsHubVisible`/`IsPremium`, `OfflineGracePeriod`,
+  `ValidateLicenseAtStartupAsync`, `ActivateLicenseAsync`).
 
 - Core file keeps: fields, ctor, `Shutdown`/`SaveLayoutNow`/`ScheduleSave`/`LoadLayout`,
   `SetPanel`/`ShowOverlay`/`ShowLicenses`, `CreateBrush`, `OpenUrl`/`ComposeEmail`,
   `SubmitBug`/`SubmitFeature`, window-geometry pass-through.
-- Partials: `Account` ✔ (Commit 1), `License` (Polar key validation + `IsPremium`),
-  `Scenes` (scene catalog + hotkeys + transitions), `Background` (background model + screen
+- Partials: `Account` ✔ (Commit 1), `License` ✔ (Commit 2), `Scenes` (scene catalog + hotkeys + transitions), `Background` (background model + screen
   capture + display picker), `Webcam` (webcam identity + per-scene configs + captures),
   `Audio` (mic + game bars + TRAX), `Trax` (music player), `Socials` (social bar + dialog),
   `Streaming` (go-live/record + health + quality tiers + media source pipeline),