Add visual feedback to bindings

[xal3xhx]

Summary

• Binding items in the list now pulse with an indigo glow when triggered by MIDI
• Glow stays active while events stream in (debounced 300ms past last event)
• Fades in/out via CSS box-shadow transition, no looping pulse

Implementation

• src/features/bindings/bindings.js — set data-binding-id on each binding item root
• src/main.js — flashBindingTrigger(id) helper called from the midi_event listener once a binding is resolved; stashes timeout on the element and clears/resets on each incoming event
• src/styles/bindings-and-panels.css — .binding-item.triggered sets a static glow; base selector has transition: box-shadow 350ms ease-out for the fade
• Dark-mode variant uses a lighter indigo (rgba(129, 140, 248, ...))

Test plan

• Move a fader bound to a volume target — binding pulses smoothly while moving, fades out ~300ms after stopping
• Press a button bound to ToggleMute / Hotkey / OpenApplication — single pulse visible
• Trigger bindings with integration targets (OBS / WaveLink) — pulse visible
• Toggle dark mode — glow color matches theme
• Rapid input on multiple bindings at once — each pulses independently