feat: add comments and cleanup unused imports across codebase

static/player/quality.ts

@@ -1,6 +1,7 @@
 import { state } from './state';
-import { displayTimeFromAbsolute, absoluteTimeFromDisplay } from './timeline';
+import { displayTimeFromAbsolute } from './timeline';
 
+// same as mode.ts - could be extracted to shared util
 const streamUrlForMode = (mode: string, quality?: string): string => {
   const src = state.modeSources[mode];
   if (!src?.token) return '';
@@ -19,6 +20,10 @@ const loadVideo = (url: string): void => {
   if (wasPlaying) state.video.play().catch(() => {});
 };
 
+/**
+ * Switches video quality (resolution).
+ * Persists preference to localStorage.
+ */
 export const switchQuality = (quality: string): void => {
   const url = streamUrlForMode(state.currentMode, quality);
   if (!url) return;
@@ -26,6 +31,10 @@ export const switchQuality = (quality: string): void => {
   loadVideo(url);
 };
 
+/**
+ * Rebuilds quality dropdown options from current mode's available qualities.
+ * Shows/hides dropdown based on availability.
+ */
 export const updateQualityOptions = (): void => {
   const select = state.container.querySelector('[data-quality-select]') as HTMLSelectElement | null;
   if (!select) return;
@@ -44,13 +53,18 @@ export const updateQualityOptions = (): void => {
     select.appendChild(opt);
   });
 
+  // restore saved preference
   const preferred = localStorage.getItem('mal:preferred-quality') || 'best';
   select.value = qualities.includes(preferred) ? preferred : 'best';
 
+  // hide if no quality options
   const wrapper = select.parentElement;
   wrapper?.classList.toggle('hidden', qualities.length === 0);
 };
 
+/**
+ * Binds quality select change handler.
+ */
 export const setupQuality = (): void => {
   const select = state.container.querySelector('[data-quality-select]') as HTMLSelectElement | null;
   select?.addEventListener('change', e => {