feat(contact): implement Formspree contact form submission

- Enable contact form fields with component IDs
- Add callback for Formspree POST with JSON/AJAX
- Include honeypot spam protection (_gotcha field)
- Handle validation, loading, success/error states
- Clear form on successful submission
- Add lessons learned documentation

Closes #92, #93, #94

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

portfolio_app/callbacks/__init__.py

@@ -1,5 +1,5 @@
 """Application-level callbacks for the portfolio app."""
 
-from . import sidebar, theme
+from . import contact, sidebar, theme
 
-__all__ = ["sidebar", "theme"]
+__all__ = ["contact", "sidebar", "theme"]

portfolio_app/callbacks/contact.py

@@ -0,0 +1,214 @@
+"""Contact form submission callback with Formspree integration."""
+
+import re
+from typing import Any
+
+import dash_mantine_components as dmc
+import requests
+from dash import Input, Output, State, callback, no_update
+from dash_iconify import DashIconify
+
+FORMSPREE_ENDPOINT = "https://formspree.io/f/mqelqzpd"
+EMAIL_REGEX = re.compile(r"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$")
+
+
+def _validate_form(
+    name: str | None, email: str | None, message: str | None
+) -> str | None:
+    """Validate form fields and return error message if invalid."""
+    if not name or not name.strip():
+        return "Please enter your name."
+    if not email or not email.strip():
+        return "Please enter your email address."
+    if not EMAIL_REGEX.match(email.strip()):
+        return "Please enter a valid email address."
+    if not message or not message.strip():
+        return "Please enter a message."
+    return None
+
+
+def _create_success_alert() -> dmc.Alert:
+    """Create success feedback alert."""
+    return dmc.Alert(
+        "Thank you for your message! I'll get back to you soon.",
+        title="Message Sent",
+        color="green",
+        variant="light",
+        icon=DashIconify(icon="tabler:check", width=20),
+        withCloseButton=True,
+    )
+
+
+def _create_error_alert(message: str) -> dmc.Alert:
+    """Create error feedback alert."""
+    return dmc.Alert(
+        message,
+        title="Error",
+        color="red",
+        variant="light",
+        icon=DashIconify(icon="tabler:alert-circle", width=20),
+        withCloseButton=True,
+    )
+
+
+@callback(  # type: ignore[misc]
+    Output("contact-feedback", "children"),
+    Output("contact-submit", "loading"),
+    Output("contact-name", "value"),
+    Output("contact-email", "value"),
+    Output("contact-subject", "value"),
+    Output("contact-message", "value"),
+    Output("contact-name", "error"),
+    Output("contact-email", "error"),
+    Output("contact-message", "error"),
+    Input("contact-submit", "n_clicks"),
+    State("contact-name", "value"),
+    State("contact-email", "value"),
+    State("contact-subject", "value"),
+    State("contact-message", "value"),
+    State("contact-gotcha", "value"),
+    prevent_initial_call=True,
+)
+def submit_contact_form(
+    n_clicks: int | None,
+    name: str | None,
+    email: str | None,
+    subject: str | None,
+    message: str | None,
+    gotcha: str | None,
+) -> tuple[Any, ...]:
+    """Submit contact form to Formspree.
+
+    Args:
+        n_clicks: Button click count.
+        name: User's name.
+        email: User's email address.
+        subject: Message subject (optional).
+        message: Message content.
+        gotcha: Honeypot field value (should be empty for real users).
+
+    Returns:
+        Tuple of (feedback, loading, name, email, subject, message,
+                  name_error, email_error, message_error).
+    """
+    if not n_clicks:
+        return (no_update,) * 9
+
+    # Check honeypot - if filled, silently "succeed" (it's a bot)
+    if gotcha:
+        return (
+            _create_success_alert(),
+            False,
+            "",
+            "",
+            None,
+            "",
+            None,
+            None,
+            None,
+        )
+
+    # Validate form
+    validation_error = _validate_form(name, email, message)
+    if validation_error:
+        # Determine which field has the error
+        name_error = "Required" if not name or not name.strip() else None
+        email_error = None
+        message_error = "Required" if not message or not message.strip() else None
+
+        if not email or not email.strip():
+            email_error = "Required"
+        elif not EMAIL_REGEX.match(email.strip()):
+            email_error = "Invalid email format"
+
+        return (
+            _create_error_alert(validation_error),
+            False,
+            no_update,
+            no_update,
+            no_update,
+            no_update,
+            name_error,
+            email_error,
+            message_error,
+        )
+
+    # Prepare form data (validation passed, so name/email/message are not None)
+    assert name is not None
+    assert email is not None
+    assert message is not None
+    form_data = {
+        "name": name.strip(),
+        "email": email.strip(),
+        "subject": subject or "General Inquiry",
+        "message": message.strip(),
+        "_gotcha": "",  # Formspree honeypot
+    }
+
+    # Submit to Formspree
+    try:
+        response = requests.post(
+            FORMSPREE_ENDPOINT,
+            json=form_data,
+            headers={
+                "Accept": "application/json",
+                "Content-Type": "application/json",
+            },
+            timeout=10,
+        )
+
+        if response.status_code == 200:
+            # Success - clear form
+            return (
+                _create_success_alert(),
+                False,
+                "",
+                "",
+                None,
+                "",
+                None,
+                None,
+                None,
+            )
+        else:
+            # Formspree returned an error
+            return (
+                _create_error_alert(
+                    "Failed to send message. Please try again or use direct contact."
+                ),
+                False,
+                no_update,
+                no_update,
+                no_update,
+                no_update,
+                None,
+                None,
+                None,
+            )
+
+    except requests.exceptions.Timeout:
+        return (
+            _create_error_alert("Request timed out. Please try again."),
+            False,
+            no_update,
+            no_update,
+            no_update,
+            no_update,
+            None,
+            None,
+            None,
+        )
+    except requests.exceptions.RequestException:
+        return (
+            _create_error_alert(
+                "Network error. Please check your connection and try again."
+            ),
+            False,
+            no_update,
+            no_update,
+            no_update,
+            no_update,
+            None,
+            None,
+            None,
+        )