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>

store/filter-store.ts

@@ -0,0 +1,50 @@
+import { create } from 'zustand';
+import type { FilterType, FilterParams, FilterState } from '@/types/filter';
+
+interface FilterStore extends FilterState {
+  setActiveFilter: (filter: FilterType | null) => void;
+  updateParams: (params: Partial<FilterParams>) => void;
+  resetParams: () => void;
+  setPreviewMode: (enabled: boolean) => void;
+}
+
+const defaultParams: FilterParams = {
+  brightness: 0,
+  contrast: 0,
+  hue: 0,
+  saturation: 0,
+  lightness: 0,
+  radius: 5,
+  amount: 50,
+  threshold: 128,
+  levels: 8,
+};
+
+export const useFilterStore = create<FilterStore>((set) => ({
+  activeFilter: null,
+  params: { ...defaultParams },
+  isPreviewMode: false,
+  presets: [],
+
+  setActiveFilter: (filter) =>
+    set((state) => ({
+      activeFilter: filter,
+      params: filter ? { ...defaultParams } : state.params,
+      isPreviewMode: false,
+    })),
+
+  updateParams: (params) =>
+    set((state) => ({
+      params: { ...state.params, ...params },
+    })),
+
+  resetParams: () =>
+    set({
+      params: { ...defaultParams },
+    }),
+
+  setPreviewMode: (enabled) =>
+    set({
+      isPreviewMode: enabled,
+    }),
+}));

store/index.ts

@@ -1,5 +1,6 @@
 export * from './canvas-store';
 export * from './layer-store';
 export * from './tool-store';
+export * from './filter-store';
 export * from './history-store';
 export * from './color-store';