feat: Consolidates staff gump layouts (#2404)
## Summary
- Creates `PropsLayoutExtensions.cs` with reusable extension methods for both legacy `Gump` and `DynamicGumpBuilder` that encapsulate the repeating PropsConfig-style layout patterns (frame, header navigation, entry rows)
- Converts all 12 standardized staff gumps to use the new extensions, reducing ~570 lines of duplicated layout code
- Adds Type and Serial display to PropsGump and SkillsGump headers (e.g. `PlayerMobile (0x1)`)
### Extension methods provided
| Method | Pattern |
|--------|---------|
| `AddPropsFrame` | Background + offset region + origin coordinates |
| `AddPropsHeader` | 3-column: [Prev] [Title] [Next] |
| `AddPropsHeaderWithBack` | 4-column: [Back] [Title] [Prev] [Next] |
| `AddPropsEntryButton` | Label + action button |
| `AddPropsEntryNameValue` | Name + Value + action button |
| `AddPropsEntryTextInput` | Text input + action button |
| `AddPropsEntryLabel` | Label only (no button) |
| `AddPropsEntryType` | Full-width type label |
| `AddPropsEntryBlank` | Separator row |
### Gumps converted
PropsGump, GoGump, WhoGump, SkillsGump (frame+header), EditSkillGump, SetGump, SetObjectGump, SetPoint2DGump, SetPoint3DGump, SetTimeSpanGump (frame), SetListOptionGump (frame+header), CategorizedAddGump (frame+header)
## Test plan
- [x] Verify `[props` gump displays correctly with type + serial in header
- [x] Verify `[skills` gump displays correctly with type + serial in header
- [x] Verify `[go` navigation gump works (prev/next/back)
- [x] Verify property editing gumps (Set, SetObject, SetPoint2D, SetPoint3D, SetTimeSpan, SetListOption)
- [x] Verify `[categorizedadd` gump works
- [x] Verify `[who` gump works with pagination
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