Refactor calendar component into modular architecture with view switching

- Split monolithic Calendar component into focused sub-components:
  - CalendarHeader: Navigation buttons and title display
  - MonthView: Monthly calendar grid layout and event rendering
  - WeekView: Weekly calendar view with full-height day containers
- Add ViewMode enum for Month/Week view switching in sidebar dropdown
- Fix event styling by correcting CSS class from "event" to "event-box"
- Implement proper week view layout with full-height day containers
- Maintain all existing functionality: event handling, context menus, localStorage persistence

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

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

src/components/calendar_header.rs

@@ -0,0 +1,54 @@
+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>,
+}
+
+#[function_component(CalendarHeader)]
+pub fn calendar_header(props: &CalendarHeaderProps) -> Html {
+    let title = match props.view_mode {
+        ViewMode::Month => {
+            format!("{} {}", get_month_name(props.current_date.month()), props.current_date.year())
+        },
+        ViewMode::Week => {
+            format!("Week of {} {}", get_month_name(props.current_date.month()), props.current_date.day())
+        },
+    };
+
+    html! {
+        <div class="calendar-header">
+            <button class="nav-button" onclick={props.on_prev.clone()}>{"‹"}</button>
+            <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"
+    }
+}