docs: remove dark mode references (not implemented)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/FEATURES.md

@@ -56,7 +56,7 @@
 12 colors matching Google Keep's palette:
 - Default, Coral, Peach, Sand, Mint, Sage
 - Fog, Storm, Dusk, Blossom, Clay, Chalk
-- Colors adapt to light/dark mode
+- Colors adapt to note theme
 
 ### Pin Notes
 - Pin important notes to always appear at top
@@ -80,13 +80,6 @@
 - Clear search to restore original view
 - Case-insensitive matching
 
-### Dark Mode
-- System preference detection (auto)
-- Manual toggle override
-- Preference persisted in localStorage
-- All components adapt to theme
-- Note colors adjust for dark backgrounds
-
 ### Single-User Auth
 - First-run setup: create password (min 8 chars)
 - Argon2 password hashing (industry standard)