feat: add 6 pastel color themes (Rose, Green, Lavender, Sand, Ocean, Sunset)

Each theme includes light and dark mode variants with carefully tuned
oklch colors for foreground, card, borders, sidebar, and all UI elements.
Total: 10 named themes + dark mode + auto (system preference).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

memento-note/app/(main)/settings/appearance/appearance-settings-client.tsx

@@ -108,6 +108,12 @@ export function AppearanceSettingsClient({ initialFontSize, initialTheme, initia
             { value: 'dark', label: t('settings.themeDark') },
             { value: 'sepia', label: 'Sepia' },
             { value: 'midnight', label: 'Midnight' },
+            { value: 'rose', label: 'Rose' },
+            { value: 'green', label: 'Green' },
+            { value: 'lavender', label: 'Lavender' },
+            { value: 'sand', label: 'Sand' },
+            { value: 'ocean', label: 'Ocean' },
+            { value: 'sunset', label: 'Sunset' },
             { value: 'auto', label: t('settings.themeSystem') },
           ]}
           onChange={handleThemeChange}