feat: support host-supplied named theme presets

[nmiddelmann2106]

Summary

Replaces the long-standing "Not Implemented" preset stub in the
ThemeManager drawer with a working dropdown driven by themes the host
application provides. The library stays storage-agnostic — it just
renders what it is given and emits a selection event.

Motivation

Issue #31 names "Add presets" as a planned improvement. There are at
least two natural shapes this can take: built-in catalog presets
(library-shipped) and host-supplied presets (loaded from app state, a
DB, JSON files, etc.). This PR addresses the second shape with a
small, additive API. It is intentionally unopinionated about
persistence: the host fills the list, the library renders and reports
selection.

Changes

• ThemeManagerTheme.cs: add optional string? Name. Existing usage
  is unaffected; only consumers that opt into presets need to set it.
• MudThemeManager.razor.cs: add two parameters
  • IEnumerable<ThemeManagerTheme>? Presets
  • EventCallback<ThemeManagerTheme> PresetSelected
• MudThemeManager.razor: when Presets is null the existing
  disabled "Not Implemented" stub still renders unchanged. When
  provided, a working MudSelect lists the names and emits the
  selection event.
• Refactor: theme initialisation moves from OnInitialized into a
  small SyncWithTheme() method called from both OnInitialized and
  OnParametersSet, guarded by a reference check on Theme. Without
  this, swapping the active Theme via the existing parameter (which
  is the natural way to react to a preset selection) would leave the
  manager's internal palette caches pointing at the previous theme,
  so subsequent edits would mutate the wrong instance.

Backward compatibility

• All new parameters are optional.
• When Presets is null the rendered drawer is byte-identical to
  before (same disabled MudSelect, same "Not Implemented" item).
• Name on ThemeManagerTheme is nullable and unused by the library
  unless the consumer opts in.

Usage

<MudThemeManager Theme="@_active"
                 Presets="@_themes"
                 PresetSelected="@OnPresetPicked"
                 ThemeChanged="@(t => _active = t)"
                 ... />

private List<ThemeManagerTheme> _themes = new();
private ThemeManagerTheme _active = new();

private void OnPresetPicked(ThemeManagerTheme picked)
{
    _active = picked;
}

Test plan

• dotnet build clean (0 errors).
• Smoke: drawer with no Presets shows the unchanged
  "Not Implemented" stub.
• Smoke: drawer with Presets shows names and switching emits
  PresetSelected.
• Smoke: switching active Theme after first render correctly
  re-syncs internal caches (subsequent color edits target the
  newly-active theme).

Notes

• This sits alongside feat: expose ColorPickerMode parameter on MudThemeManager #41 (ColorPickerMode parameter) but is
  independent. Either can land in any order.
• Re ThemeManager planned improvements, feedback wanted #31: deliberately scoped to the host-supplied shape only.
  Built-in catalog presets and import/export from JSON are separate
  concerns and can compose naturally with this API.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud