Add interactive getting-started tutorial for first-time users

Auto-starts on first visit with a 9-step walkthrough covering header,
tabs, settings, search, theme, and language controls. Stores completion
in localStorage; restart via ? button in the header. Includes en/ru/zh
translations for all tour steps.

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

server/src/wled_controller/static/locales/en.json

@@ -214,6 +214,16 @@
   "calibration.tip.border_width": "How many pixels from the screen edge to sample for LED colors",
   "calibration.tip.skip_leds_start": "Skip LEDs at the start of the strip — skipped LEDs stay off",
   "calibration.tip.skip_leds_end": "Skip LEDs at the end of the strip — skipped LEDs stay off",
+  "tour.welcome": "Welcome to LED Grab! This quick tour will show you around the interface. Use arrow keys or buttons to navigate.",
+  "tour.dashboard": "Dashboard — live overview of running targets, profiles, and device health at a glance.",
+  "tour.targets": "Targets — add WLED devices, configure LED targets with capture settings and calibration.",
+  "tour.sources": "Sources — manage capture templates, picture sources, audio sources, and color strips.",
+  "tour.profiles": "Profiles — group targets and automate switching with time, audio, or value conditions.",
+  "tour.settings": "Settings — backup and restore configuration, manage auto-backups.",
+  "tour.search": "Search — quickly find and navigate to any entity with Ctrl+K.",
+  "tour.theme": "Theme — switch between dark and light mode.",
+  "tour.language": "Language — choose your preferred interface language.",
+  "tour.restart": "Restart tutorial",
   "calibration.tutorial.start": "Start tutorial",
   "calibration.overlay_toggle": "Overlay",
   "calibration.start_position": "Starting Position:",