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.

jetc/build-assets.sh

@@ -0,0 +1,52 @@
+#!/bin/bash
+# Builds the hand-written assets.compiled.cpp module into assets.jtc.
+# Mirrors build-shared.sh's compile/link tail exactly (same object cache,
+# same libCoreUtils_pic.a, same link line) but skips the jetc --shared
+# codegen step since there is no .jet source for this module.
+set -e
+JC="$(cd "/home/barant/servercore-analysis/JetCore" && pwd)"
+CU="$JC/../CoreUtils"
+HERE="$JC/jetc"
+STEM="assets"
+OUTPUT="$1"
+
+if [ -z "$OUTPUT" ]; then
+   echo "usage: $0 <output.jtc>"
+   exit 1
+fi
+
+echo "Building JetCore runtime objects as position-independent code..."
+list=""
+for f in "$JC"/*.cpp; do
+   base=$(basename "$f")
+   if [ "$base" = "jet-2.0.cpp" ]; then continue; fi
+   obj="$HERE/${base%.cpp}.pic.o"
+   if [ ! -f "$obj" ] || [ "$f" -nt "$obj" ]; then
+      g++ -g -fPIC -c -std=c++23 -I"$CU" -I"$JC" "$f" -o "$obj" &
+   fi
+   list="$list $obj"
+done
+
+cu_pic_list=""
+cu_rebuilt=false
+for f in "$CU"/*.cpp; do
+   base=$(basename "$f")
+   obj="$HERE/cu_${base%.cpp}.pic.o"
+   if [ ! -f "$obj" ] || [ "$f" -nt "$obj" ]; then
+      g++ -g -fPIC -c -std=c++23 -I"$CU" "$f" -o "$obj" &
+      cu_rebuilt=true
+   fi
+   cu_pic_list="$cu_pic_list $obj"
+done
+wait
+if [ "$cu_rebuilt" = true ] || [ ! -f "$HERE/libCoreUtils_pic.a" ]; then
+   ar rcs "$HERE/libCoreUtils_pic.a" $cu_pic_list
+fi
+
+g++ -g -fPIC -c -std=c++23 -I"$CU" -I"$JC" "$HERE/$STEM.compiled.cpp" -o "$HERE/$STEM.compiled.pic.o"
+
+echo "Linking $OUTPUT..."
+g++ -shared -fPIC -o "$OUTPUT" "$HERE/$STEM.compiled.pic.o" $list -std=c++23 \
+   -L"$HERE" -lCoreUtils_pic -lmysqlclient -lcrypto
+
+echo "Built $OUTPUT"