feat: add core application constants, ownership verification, and repository utilities

- Introduced application-wide constants for file uploads, image processing, pagination, and authentication in `constants.py`.
- Implemented synchronous and asynchronous board ownership verification functions in `ownership.py`.
- Created a base repository class with common CRUD operations in `repository.py`.
- Added standard response utilities for error and success messages in `responses.py`.
- Refactored image validation to utilize constants for file size and MIME types.
- Enhanced frontend components with consistent styling and validation utilities for forms.
- Established global styles for buttons, forms, loading indicators, and messages to ensure a cohesive UI experience.

frontend/src/lib/styles/form.css

@@ -0,0 +1,75 @@
+/**
+ * Shared form styles
+ * Used across all forms to maintain consistency and reduce duplication
+ */
+
+/* Form containers */
+.form-group {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+  margin-bottom: 1.5rem;
+}
+
+/* Labels */
+.form-label {
+  display: block;
+  font-weight: 500;
+  color: #374151;
+  margin-bottom: 0.5rem;
+  font-size: 0.95rem;
+}
+
+.form-label .required {
+  color: #ef4444;
+}
+
+/* Inputs and textareas */
+.form-input,
+.form-textarea {
+  width: 100%;
+  padding: 0.75rem;
+  border: 1px solid #d1d5db;
+  border-radius: 8px;
+  font-size: 1rem;
+  font-family: inherit;
+  transition: all 0.2s;
+}
+
+.form-input:focus,
+.form-textarea:focus {
+  outline: none;
+  border-color: #667eea;
+  box-shadow: 0 0 0 3px rgba(102, 126, 234, 0.1);
+}
+
+.form-input.error,
+.form-textarea.error {
+  border-color: #ef4444;
+}
+
+.form-input:disabled,
+.form-textarea:disabled {
+  background-color: #f9fafb;
+  cursor: not-allowed;
+}
+
+.form-textarea {
+  resize: vertical;
+  min-height: 100px;
+}
+
+/* Help and error text */
+.form-help-text {
+  display: block;
+  color: #6b7280;
+  font-size: 0.875rem;
+  margin-top: 0.25rem;
+}
+
+.form-error-text {
+  display: block;
+  color: #ef4444;
+  font-size: 0.875rem;
+  margin-top: 0.25rem;
+}