feat: CalDAV sync adapter with calendar discovery and delta sync

Add CalDAV protocol implementation: principal discovery, calendar-home-set
lookup, calendar listing, and sync-collection delta sync. CalDAVSync parses
iCalendar events from CalDAV resources and populates the sync cache.
CalDAVSyncProvider implements SyncProvider for account-level orchestration.

include/nocal/sync/caldav_account.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <string>
+#include <string_view>
+
+namespace nocal::sync {
+
+// CalDAV account configuration.
+struct CalDAVAccountConfig {
+    std::string server_url;
+    std::string username;
+    std::string password;
+    std::string account_id;
+    std::string display_name;
+};
+
+// Derive a deterministic account ID from server URL and username.
+[[nodiscard]] std::string caldav_account_id(
+    std::string_view server_url, std::string_view username);
+
+} // namespace nocal::sync

include/nocal/sync/caldav_sync.hpp

@@ -0,0 +1,52 @@
+#pragma once
+
+#include "nocal/sync/caldav_account.hpp"
+#include "nocal/sync/http.hpp"
+#include "nocal/storage/sync_cache.hpp"
+
+#include <string>
+#include <vector>
+
+namespace nocal::sync {
+
+// CalDAV calendar listing entry.
+struct CalDAVCalendar {
+    std::string href;
+    std::string name;
+    std::string description;
+    bool read_only{false};
+};
+
+class CalDAVSync {
+public:
+    CalDAVSync(HttpTransport& transport, storage::SyncCache& cache,
+               const CalDAVAccountConfig& config);
+
+    // Discover principal URL and calendar-home-set.
+    void discover();
+
+    // List calendars under the calendar-home-set.
+    std::vector<CalDAVCalendar> list_calendars();
+
+    // Sync one calendar using sync-collection (delta) or full PROPFIND.
+    // `sync_token` is empty for initial sync. Returns new sync token.
+    std::string sync_calendar(const CalDAVCalendar& calendar,
+                              std::string_view sync_token);
+
+private:
+    HttpTransport& transport_;
+    storage::SyncCache& cache_;
+    CalDAVAccountConfig config_;
+
+    std::string principal_url_;
+    std::string calendar_home_set_;
+
+    std::string send_request(int method, const std::string& url,
+                             const std::string& body, const std::vector<HttpHeader>& extra_headers);
+    std::string calendar_resource_url(const std::string& calendar_href) const;
+    std::string stable_calendar_id(const std::string& remote_id) const;
+    std::string stable_event_id(const std::string& calendar_id, const std::string& remote_uid) const;
+    std::string encode_basic_auth() const;
+};
+
+} // namespace nocal::sync

include/nocal/sync/caldav_sync_provider.hpp

@@ -0,0 +1,31 @@
+#pragma once
+
+#include "nocal/sync/caldav_account.hpp"
+#include "nocal/sync/http.hpp"
+#include "nocal/sync/provider.hpp"
+
+namespace nocal::storage {
+class SyncCache;
+}
+
+#include <string>
+
+namespace nocal::sync {
+
+class CalDAVSyncProvider final : public SyncProvider {
+public:
+    CalDAVSyncProvider(HttpTransport& transport, storage::SyncCache& cache,
+                       CalDAVAccountConfig config);
+
+    std::string_view provider_id() const noexcept override { return "caldav"; }
+    ConnectedAccount connect_account() override;
+    void sync_account(std::string_view account_id, SyncObserver& observer) override;
+    void disconnect_account(std::string_view account_id) override;
+
+private:
+    HttpTransport& transport_;
+    storage::SyncCache& cache_;
+    CalDAVAccountConfig config_;
+};
+
+} // namespace nocal::sync