feat(phase-7): implement comprehensive effects & filters system

This commit completes Phase 7 of the paint-ui implementation, adding a
complete filters and effects system with live preview capabilities.

**New Files:**
- types/filter.ts: Filter types, parameters, and state interfaces
- lib/filter-utils.ts: Core filter algorithms and image processing functions
- core/commands/filter-command.ts: Undo/redo support for filters
- store/filter-store.ts: Filter state management with Zustand
- hooks/use-filter-preview.ts: Real-time filter preview system
- components/filters/filter-panel.tsx: Complete filter UI with parameters
- components/filters/index.ts: Filters barrel export

**Updated Files:**
- components/editor/editor-layout.tsx: Integrated FilterPanel into layout
- store/index.ts: Added filter-store export
- types/index.ts: Added filter types export

**Implemented Filters:**

**Adjustment Filters (with parameters):**
- ✨ Brightness (-100 to +100): Linear brightness adjustment
- ✨ Contrast (-100 to +100): Contrast curve adjustment
- ✨ Hue/Saturation/Lightness: Full HSL color manipulation
  - Hue: -180° to +180° rotation
  - Saturation: -100% to +100% adjustment
  - Lightness: -100% to +100% adjustment

**Effect Filters (with parameters):**
- ✨ Gaussian Blur (1-50px): Separable kernel blur with proper edge handling
- ✨ Sharpen (0-100%): Unsharp mask algorithm
- ✨ Threshold (0-255): Binary threshold conversion
- ✨ Posterize (2-256 levels): Color quantization

**One-Click Filters (no parameters):**
- ✨ Invert: Color inversion
- ✨ Grayscale: Luminosity-based desaturation
- ✨ Sepia: Classic sepia tone effect

**Technical Features:**
- Real-time preview system with toggle control
- Non-destructive preview (restores original on cancel)
- Undo/redo integration via FilterCommand
- Efficient image processing with typed arrays
- HSL/RGB color space conversions
- Separable Gaussian blur for performance
- Proper clamping and edge case handling
- Layer-aware filtering (respects locked layers)

**UI/UX Features:**
- 264px wide filter panel with all filters listed
- Dynamic parameter controls based on selected filter
- Live preview toggle with visual feedback
- Apply/Cancel actions with proper state cleanup
- Disabled state when no unlocked layer selected
- Clear parameter labels and value display

**Algorithm Implementations:**
- Brightness: Linear RGB adjustment with clamping
- Contrast: Standard contrast curve (factor-based)
- Hue/Saturation: Full RGB↔HSL conversion with proper hue rotation
- Blur: Separable Gaussian kernel (horizontal + vertical passes)
- Sharpen: Convolution kernel with configurable amount
- Threshold: Luminosity-based binary conversion
- Posterize: Color quantization with configurable levels

Build verified: ✓ Compiled successfully in 1248ms

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

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

components/editor/editor-layout.tsx

@@ -9,6 +9,7 @@ import { HistoryPanel } from './history-panel';
 import { FileMenu } from './file-menu';
 import { ToolPalette, ToolSettings } from '@/components/tools';
 import { ColorPanel } from '@/components/colors';
+import { FilterPanel } from '@/components/filters';
 import { useKeyboardShortcuts } from '@/hooks/use-keyboard-shortcuts';
 import { useFileOperations } from '@/hooks/use-file-operations';
 import { useDragDrop } from '@/hooks/use-drag-drop';
@@ -175,6 +176,9 @@ export function EditorLayout() {
         {/* Color Panel */}
         <ColorPanel />
 
+        {/* Filter Panel */}
+        <FilterPanel />
+
         {/* Canvas area */}
         <div className="flex-1">
           <CanvasWithTools />