feat: refine UI with effects panel improvements and visual polish

Major improvements:
- Fixed multi-file import (FileList to Array conversion)
- Auto-select first track when adding to empty project
- Global effects panel folding state (independent of track selection)
- Effects panel collapsed/disabled when no track selected
- Effect device expansion state persisted per-device
- Effect browser with searchable descriptions

Visual refinements:
- Removed center dot from pan knob for cleaner look
- Simplified fader: removed volume fill overlay, dynamic level meter visible through semi-transparent handle
- Level meter capped at fader position (realistic mixer behavior)
- Solid background instead of gradient for fader track
- Subtle volume overlay up to fader handle
- Fixed track control width flickering (consistent 4px border)
- Effect devices: removed shadows/rounded corners for flatter DAW-style look, added consistent border-radius
- Added border between track control and waveform area

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/ui/CircularKnob.tsx

@@ -167,11 +167,6 @@ export function CircularKnob({
           {/* Indicator line */}
           <div className="absolute top-1 left-1/2 w-0.5 h-2 bg-primary rounded-full -translate-x-1/2" />
         </div>
-
-        {/* Center dot (for zero position) */}
-        {value === 0 && (
-          <div className="absolute top-1/2 left-1/2 w-1 h-1 bg-primary rounded-full -translate-x-1/2 -translate-y-1/2" />
-        )}
       </div>
 
       {/* Value Display */}