feat(settings): custom accent color picker for non-TDS themes (P5-1)

Adds a customAccentColor setting + a HexColorPickerPopOut in Settings →
Appearance. When set (and Lotus Terminal/TDS is OFF), it derives a full folds
Primary palette (Main/hover/active/line, contrasting OnMain, alpha-tiered
Container set, OnContainer) from the chosen color and overrides the folds
Primary CSS variables on document.body — resolving each var name from the
imported folds color.Primary.* token strings (e.g. "var(--oq6d07f)"), the
same body-level injection pattern used for mentionHighlightColor. The theme
class is on document.body, so an inline override on body wins over it.
Reverts to theme defaults when unset or when Lotus Terminal is enabled (TDS
keeps its fixed palette); the picker is disabled with a note in TDS mode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/settings.ts

@@ -146,6 +146,7 @@ export interface Settings {
   composerToolbarButtons: ComposerToolbarSettings;
 
   mentionHighlightColor: string;
+  customAccentColor: string;
   fontFamily: 'system' | 'inter' | 'jetbrains-mono' | 'fira-code';
 
   afkAutoMute: boolean;
@@ -242,6 +243,7 @@ const defaultSettings: Settings = {
   composerToolbarButtons: DEFAULT_COMPOSER_TOOLBAR,
 
   mentionHighlightColor: '',
+  customAccentColor: '',
   fontFamily: 'inter',
 
   afkAutoMute: false,