registering canvas functions now instead of internal state

client/docs/canvas_add_functionality.md

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+## How Canvas Library Custom Functionality Works
+
+### 1. **The Canvas Library Calls YOUR Custom Code First**
+
+When you call `ActionDispatcher::dispatch()`, here's what happens:
+
+```rust
+// Inside canvas library (canvas/src/actions/edit.rs):
+pub async fn execute_canvas_action<S: CanvasState>(
+    action: CanvasAction,
+    state: &mut S,
+    ideal_cursor_column: &mut usize,
+) -> Result<ActionResult> {
+    // 1. FIRST: Canvas library calls YOUR custom handler
+    if let Some(result) = state.handle_feature_action(&action, &context) {
+        return Ok(ActionResult::HandledByFeature(result)); // YOUR code handled it
+    }
+
+    // 2. ONLY IF your code returns None: Canvas handles generic actions
+    handle_generic_canvas_action(action, state, ideal_cursor_column).await
+}
+```
+
+### 2. **Your Extension Point: `handle_feature_action`**
+
+You add custom functionality by implementing `handle_feature_action` in your states:
+
+```rust
+// In src/state/pages/auth.rs
+impl CanvasState for LoginState {
+    // ... other methods ...
+
+    fn handle_feature_action(&mut self, action: &CanvasAction, context: &ActionContext) -> Option<String> {
+        match action {
+            // Custom login-specific actions
+            CanvasAction::Custom(action_str) if action_str == "submit_login" => {
+                if self.username.is_empty() || self.password.is_empty() {
+                    Some("Please fill in all required fields".to_string())
+                } else {
+                    // Trigger login process
+                    Some(format!("Logging in user: {}", self.username))
+                }
+            }
+            
+            CanvasAction::Custom(action_str) if action_str == "clear_form" => {
+                self.username.clear();
+                self.password.clear();
+                self.set_has_unsaved_changes(false);
+                Some("Login form cleared".to_string())
+            }
+
+            // Custom behavior for standard actions
+            CanvasAction::NextField => {
+                // Custom validation when moving between fields
+                if self.current_field == 0 && self.username.is_empty() {
+                    Some("Username cannot be empty".to_string())
+                } else {
+                    None // Let canvas library handle the normal field movement
+                }
+            }
+
+            // Let canvas library handle everything else
+            _ => None,
+        }
+    }
+}
+```
+
+### 3. **Multiple Ways to Add Custom Functionality**
+
+#### A) **Custom Actions via Config**
+```toml
+# In config.toml
+[keybindings.edit]
+submit_login = ["ctrl+enter"]
+clear_form = ["ctrl+r"]
+```
+
+#### B) **Override Standard Actions**
+```rust
+fn handle_feature_action(&mut self, action: &CanvasAction, context: &ActionContext) -> Option<String> {
+    match action {
+        CanvasAction::InsertChar('p') if self.current_field == 1 => {
+            // Custom behavior when typing 'p' in password field
+            Some("Password field - use secure input".to_string())
+        }
+        _ => None, // Let canvas handle normally
+    }
+}
+```
+
+#### C) **Context-Aware Logic**
+```rust
+fn handle_feature_action(&mut self, action: &CanvasAction, context: &ActionContext) -> Option<String> {
+    match action {
+        CanvasAction::MoveDown => {
+            // Custom logic based on current state
+            if context.current_field == 1 && context.current_input.len() < 8 {
+                Some("Password should be at least 8 characters".to_string())
+            } else {
+                None // Normal field movement
+            }
+        }
+        _ => None,
+    }
+}
+```
+
+## The Canvas Library Philosophy
+
+**Canvas Library = Generic behavior + Your extension points**
+
+- ✅ **Canvas handles**: Character insertion, cursor movement, field navigation, etc.
+- ✅ **You handle**: Validation, submission, clearing, app-specific logic
+- ✅ **You decide**: Return `Some(message)` to override, `None` to use canvas default
+
+## Summary
+
+You **don't communicate with the library elsewhere**. Instead:
+
+1. **Canvas library calls your code first** via `handle_feature_action`
+2. **Your code decides** whether to handle the action or let canvas handle it
+3. **Canvas library handles** generic form behavior when you return `None`
+