Implement shared RFC 5545 VEvent library with workspace restructuring

- Created calendar-models/ shared library with RFC 5545-compliant VEvent structures
- Migrated backend to use shared VEvent with proper field mappings (dtstart/dtend, rrule, exdate, etc.)
- Converted CalDAV client to parse into VEvent structures with structured types
- Updated all CRUD handlers to use VEvent with CalendarUser, Attendee, VAlarm types
- Restructured project as Cargo workspace with frontend/, backend/, calendar-models/
- Updated Trunk configuration for new directory structure
- Fixed all compilation errors and field references throughout codebase
- Updated documentation and build instructions for workspace structure

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/components/calendar_header.rs

@@ -0,0 +1,64 @@
+use yew::prelude::*;
+use chrono::{NaiveDate, Datelike};
+use crate::components::ViewMode;
+use web_sys::MouseEvent;
+
+#[derive(Properties, PartialEq)]
+pub struct CalendarHeaderProps {
+    pub current_date: NaiveDate,
+    pub view_mode: ViewMode,
+    pub on_prev: Callback<MouseEvent>,
+    pub on_next: Callback<MouseEvent>,
+    pub on_today: Callback<MouseEvent>,
+    #[prop_or_default]
+    pub time_increment: Option<u32>,
+    #[prop_or_default]
+    pub on_time_increment_toggle: Option<Callback<MouseEvent>>,
+}
+
+#[function_component(CalendarHeader)]
+pub fn calendar_header(props: &CalendarHeaderProps) -> Html {
+    let title = format!("{} {}", get_month_name(props.current_date.month()), props.current_date.year());
+
+    html! {
+        <div class="calendar-header">
+            <div class="header-left">
+                <button class="nav-button" onclick={props.on_prev.clone()}>{"‹"}</button>
+                {
+                    if let (Some(increment), Some(callback)) = (props.time_increment, &props.on_time_increment_toggle) {
+                        html! {
+                            <button class="time-increment-button" onclick={callback.clone()}>
+                                {format!("{}", increment)}
+                            </button>
+                        }
+                    } else {
+                        html! {}
+                    }
+                }
+            </div>
+            <h2 class="month-year">{title}</h2>
+            <div class="header-right">
+                <button class="today-button" onclick={props.on_today.clone()}>{"Today"}</button>
+                <button class="nav-button" onclick={props.on_next.clone()}>{"›"}</button>
+            </div>
+        </div>
+    }
+}
+
+fn get_month_name(month: u32) -> &'static str {
+    match month {
+        1 => "January",
+        2 => "February", 
+        3 => "March",
+        4 => "April",
+        5 => "May",
+        6 => "June",
+        7 => "July",
+        8 => "August",
+        9 => "September",
+        10 => "October",
+        11 => "November",
+        12 => "December",
+        _ => "Invalid"
+    }
+}