feat: add email templates with placeholder variables and composer integration

- Reusable email templates with local storage persistence
- Dynamic placeholder variables ({{recipientName}}, {{date}}, etc.) with auto-fill
- Template manager modal with category filtering and search
- Template picker integrated in composer toolbar (Ctrl+Shift+T)
- Settings tab for template management
- 48 unit tests for template utilities
- i18n support for all 8 languages

components/keyboard-shortcuts-modal.tsx

@@ -135,6 +135,22 @@ export function KeyboardShortcutsModal({ isOpen, onClose }: KeyboardShortcutsMod
                 ))}
               </div>
             </section>
+
+            {/* Composer Section */}
+            <section className="md:col-span-2">
+              <h3 className="text-sm font-semibold text-foreground mb-3 uppercase tracking-wider">
+                {t("shortcuts.sections.composer")}
+              </h3>
+              <div className="grid grid-cols-1 md:grid-cols-2 gap-2">
+                {KEYBOARD_SHORTCUTS.composer.map((shortcut) => (
+                  <ShortcutRow
+                    key={shortcut.key}
+                    shortcutKey={shortcut.key}
+                    description={t(shortcut.description)}
+                  />
+                ))}
+              </div>
+            </section>
           </div>
 
           {/* Footer tip */}