Declare in corresponding header files and dissolve main.h (#3796)

* Declare in corresponding header files and dissolve main.h

Apply include-what-you-use

* Remove further unncessary includes

* Incorporate review comment

* Do not declare static functions and variables in header

* Adapt test

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/recur.h

@@ -0,0 +1,57 @@
+////////////////////////////////////////////////////////////////////////////////
+//
+// Copyright 2006 - 2025, Tomas Babej, Paul Beckingham, Federico Hernandez,
+// 						  Tobias Predel.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+// THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+//
+// https://www.opensource.org/licenses/mit-license.php
+//
+////////////////////////////////////////////////////////////////////////////////
+
+#ifndef INCLUDED_RECUR
+#define INCLUDED_RECUR
+
+#include <cmake.h>
+// cmake.h include header must come first
+
+#include <Context.h>
+#include <Datetime.h>
+#include <Duration.h>
+#include <Lexer.h>
+#include <format.h>
+#include <pwd.h>
+#include <sys/types.h>
+#include <time.h>
+#include <unicode.h>
+#include <unistd.h>
+#include <util.h>
+
+#include <optional>
+
+std::optional<Datetime> checked_add_datetime(Datetime& base, time_t delta);
+void handleRecurrence();
+bool generateDueDates(Task& parent, std::vector<Datetime>& allDue);
+std::optional<Datetime> getNextRecurrence(Datetime& current, std::string& period);
+void updateRecurrenceMask(Task& task);
+void handleUntil();
+
+#endif
+
+////////////////////////////////////////////////////////////////////////////////