feat(text): add static text editor and preview

TextOverlay layers now have a multiline editor, Windows font and size controls, color/emphasis/alignment settings, and a visible draggable preview. Text sources follow OBS's static text-source conventions: https://obsproject.com/kb/text-sources

ViewModels/index.md

@@ -29,7 +29,7 @@ areas' docs in their own rows keeps this index small to read.
 | `MicPickerViewModel.cs` | Select Microphone dialog (voice source): async mic list (loading / has / none states), `UseRequested(MicrophoneDeviceInfo)`/`CancelRequested` |
 | `SocialsDialogViewModel.cs` | Social Media Site Promotion dialog (six fixed slots, `SocialSlotViewModel`): row 0 = YouTube sign-in gate (sign-in provider fills the channel, delete = sign-out); rows 1-5 validated entries / empty / freemium locks 2-5 (`isPremium` seam); `DetectService` → `ISocialValidator` on confirm (input snaps to text + real logo on success, red do-not + stays editable on failure); `CanSave` blocked while any slot edits/validates/errors; `BarEnabled` switch; commit via `CommittedEntries` + `BuildSocialsConfig()` (YouTube first) + `SaveRequested`/`CancelRequested`. WPF-free — the window owns the dialogs (delete confirm, sign-in gate) and calls `StartEdit`/`ConfirmEdit`/`DeleteSlotAsync`. |
 
-| `MainViewModel.Sources.cs` | Source creation/removal; `AddSource` creates the standalone ticker layer with one-per-scene gating and its 1920×48 default geometry. |
+| `MainViewModel.Sources.cs` | Source creation/removal; `AddSource` creates the standalone ticker layer with one-per-scene gating and its 1920×48 default geometry; TextOverlay starts as a visible 900×140 draggable text layer. |
 | `MainViewModel.AlertTicker.cs` | AlertBox ticker preview routing: selects the preview scene's AlertBox settings and publishes the positioned box-sized frame to PreviewPane. |
 
 Related: [`Models/index.md`](../Models/index.md) for the data; [`Services/index.md`](../Services/index.md)