Fix parallel I2S DMA buffer estimation (8x multiplier)

Fixed critical bug in I2S memory estimation where parallel I2S wasn't accounting for 8x larger DMA buffer:

**Problem:**
- Parallel I2S uses 8 channels simultaneously (I2S0-I2S7)
- Each channel requires its own DMA buffer
- Total DMA buffer is 8x larger than single I2S mode
- Both UI and firmware were underestimating memory by 8x

**Changes:**

1. **FX_fcn.cpp (firmware estimation):**
   - Added 8x multiplier when `hasParallelOutput()` is true
   - Applied after calculating base i2sCommonSize
   - Only affects parallel I2S mode, single I2S unchanged

2. **settings_leds.htm (UI estimation):**
   - Added check for parallel I2S checkbox (`d.Sf.PI.checked`)
   - Multiplies DMA buffer size by 8 when parallel mode enabled
   - Provides accurate memory estimate to users before saving

**Example Impact:**
- Single I2S with 300 LEDs: ~2.7KB DMA buffer
- Parallel I2S with 300 LEDs: ~21.6KB DMA buffer (8x)
- Without fix: UI showed 2.7KB, firmware allocated 21.6KB → confusion
- With fix: Both correctly show and allocate 21.6KB

**Technical Details:**
- Parallel I2S (X8 methods) drive 8 output pins simultaneously
- Each pin needs independent DMA buffer for timing
- NeoPixelBus library allocates 8 separate buffers internally
- Memory estimation must match actual allocation

**Testing:**
- ✅ Web UI built successfully
- ✅ All 16 npm tests passed
- ✅ No compilation errors
- ✅ Accurate memory estimation for both modes

Co-authored-by: DedeHai <6280424+DedeHai@users.noreply.github.com>

Author: Copilot

wled00/FX_fcn.cpp

@@ -1234,6 +1234,8 @@ void WS2812FX::finalizeInit() {
       constexpr unsigned stepFactor = 3; // 3 step cadence (3 bits per pixel bit)
       #endif
       unsigned i2sCommonSize = stepFactor * bus.count * (3*Bus::hasRGB(bus.type)+Bus::hasWhite(bus.type)+Bus::hasCCT(bus.type)) * (Bus::is16bit(bus.type)+1);
+      // Parallel I2S uses 8 channels, requiring 8x the DMA buffer size
+      if (BusManager::hasParallelOutput()) i2sCommonSize *= 8;
       if (i2sCommonSize > maxI2S) maxI2S = i2sCommonSize;
     }
     #endif

wled00/data/settings_leds.htm

@@ -240,6 +240,8 @@
 					mul = 2; // RMT uses double buffer
 				} else if (enabledI2S && toNum(n) < 8) { // I2S/LCD uses extra DMA buffer
 					dbl = len * ch * 3; // DMA buffer for I2S/LCD (TODO: only the bus with largest LED count should be used)
+					// Parallel I2S uses 8 channels, requiring 8x the DMA buffer size
+					if (d.Sf.PI && d.Sf.PI.checked) dbl *= 8;
 				}
 			}
 			return len * ch * mul + dbl + pbfr;