feat: add tag color customization with hex picker

- Add nullable color column to tags table with hex validation
- Build ColorPicker component with preset palette and custom hex input
- Apply tag colors to project cards via border styling
- Update all tag API endpoints to handle color field

migrations/20260107000447_add_tag_color.sql

@@ -0,0 +1,9 @@
+-- Add color column to tags table (nullable hex color without hash)
+ALTER TABLE tags ADD COLUMN color VARCHAR(6) DEFAULT NULL;
+
+-- Add check constraint for valid hex format (6 characters, 0-9a-fA-F)
+ALTER TABLE tags ADD CONSTRAINT tags_color_hex_format 
+  CHECK (color IS NULL OR color ~ '^[0-9a-fA-F]{6}$');
+
+-- Create index for color lookups (optional, for future filtering)
+CREATE INDEX idx_tags_color ON tags(color) WHERE color IS NOT NULL;