Add IMAP webmail tag, admin/opensource subpage tiers, store/cart, htmleditor, uploads

New __imap*/__smtpsend/__eval Jet tags give templates real IMAP
(list/message/flag/delete/move/search/folders/accounts) and SMTP-send
capability, backing the new /email webmail client (email.jet/
email.menu, assets/images/email.js + newemail.js) and its
mainpagetemplate.jet-based page shell. Most of this tag family's
source was already being built into the live site (Tag.cpp already
registered __imap/__imaplist/__imapmessage/__imapflag/__imapdelete/
__eval) but had never actually been committed; this also registers
and commits the remaining five (__imapmove/__imapfolders/
__imapsearch/__smtpsend/__imapaccounts).

admin.page/admin.menu gain a real subpage tier (admin-email,
admin-newemail, admin-invoices, admin-products, admin-users) replacing
several previously top-level pages, mirrored by a similar
opensource.page subpage tier (opensource-overview/asm1802/barantmail/
jet/serverframework/synthmax) - old top-level pages retired via git
rename (asm1802.page, jet.page, serverframework.page,
admin_invoices.page); barantmail.page is dropped outright, folded into
opensource-barantmail.subpage.

New storefront (store.page, store-createstore.subpage,
barantstore.page) and cart flow (cart.jet, updatecartqty.proc,
removecartitem.proc, editproduct.page/selectproduct.proc/
update_product.proc for inventory maintenance against the barant
database).

New document upload/download manager (documents.jet, documents_api.jet,
documents_upload.proc, documents_delete.proc,
assets/images/documents.js + documents-page.js) with a persistent
floating panel, and a rich-text page editor (htmleditor.jet/.page,
createhtmleditor.proc, savehtmleditor.proc, assets/images/htmleditor.js
+ its icon/logo assets).

SESSIONCTL.tag: login now redirects back to the originally-requested
page instead of always landing on /account ($[start:0] vs $[start]
variable-name mismatch).

main.css/menu.bar/invoices.menu/jet_overview.section/login.menu:
styling and navigation updates for the above; new site assets (fonts,
images, audio) referenced by them.

jetc/build-assets.sh, list_jet_files.sh, lowercase_jet.py,
strip_fragments.py: small dev tooling used while building the above.

__imapaccounts.h

@@ -0,0 +1,59 @@
+#ifndef ____imapaccounts_h__
+# define ____imapaccounts_h__
+
+# include "Tag.h"
+# include "ZString.h"
+# include "MString.h"
+# include <string>
+# include <utility>
+# include <vector>
+
+namespace jet {
+
+   ///
+   /// <imapaccounts action="add|remove|getpassword|firstuser|list"
+   ///                list="$[@_imap_accounts]" user="..." password="...">
+   ///
+   /// Pure list-transform tag (no session-variable access of its own,
+   /// no IMAP connection) that manages the multi-account list webmail
+   /// keeps in the single session variable "_imap_accounts" - the .jet
+   /// route wraps each call in <eval name="_imap_accounts"> itself and
+   /// <set>s it back, the same separation-of-concerns every other
+   /// session variable in this codebase already uses (this tag never
+   /// touches Global's session-variable map directly).
+   ///
+   /// The list's own on-the-wire encoding (semicolon-separated
+   /// "base64(user):base64(password)" entries) is private to this tag -
+   /// callers only ever pass the whole `list` value back in verbatim
+   /// and use the returned value the same way, so the choice of
+   /// delimiter can't collide with any real username or password
+   /// character.
+   ///
+   /// action="add": upserts user/password (removing any existing entry
+   /// for the same user first, so reconnecting with a changed password
+   /// updates it) - returns the updated encoded list.
+   /// action="remove": drops the entry for user, if present - returns
+   /// the updated encoded list.
+   /// action="getpassword": returns the stored password for user, or
+   /// an empty string if not present.
+   /// action="firstuser": returns the first entry's username, or an
+   /// empty string if the list is empty.
+   /// action="list": returns JSON ({"accounts":[...],"active":"..."})
+   /// - the one client-facing action; "active" echoes the `active`
+   /// keyword (typically $[@_imap_user]) back verbatim so the sidebar
+   /// can highlight the current one, and never includes any password.
+   ///
+   class __imapaccounts : public Tag {
+
+    public:
+      __imapaccounts(coreutils::ZString &in, coreutils::MString &parentOut, Global &global, Tag *parent, Tag *local);
+
+    private:
+      static std::vector<std::pair<std::string, std::string>> decodeList(const std::string &encoded);
+      static std::string encodeList(const std::vector<std::pair<std::string, std::string>> &entries);
+      static std::string jsonEscape(const std::string &s);
+   };
+
+}
+
+#endif