security: fix unauthenticated file upload/RCE risk, TLS bypass, XSS, and broken gift-card/review columns

- admin/api/upload-splash.php had NO admin-auth check (included the public
  customer header, not admin/includes/header.php) and both upload endpoints
  trusted the client-supplied MIME type and filename extension, so an
  attacker could name a file "shell.php", spoof Content-Type: image/png,
  and get PHP written into a web-reachable uploads/ directory. Added
  AdminAuth check and centralized real-content validation (getimagesize +
  server-side extension mapping) in a new handleImageUpload() helper used
  by both admin/upload-image.php and admin/api/upload-splash.php.
- Removed CURLOPT_SSL_VERIFYPEER => false from the CyberMail email calls
  in includes/email.php and includes/functions.php (MITM risk on the API
  key). Rewrote functions.php's sendEmail() as a thin wrapper around
  Email::send() so there's one implementation instead of two that could
  drift (this is what had the second copy of the TLS bypass).
- Escaped customer-controlled fields (customer_name, tracking info, reset
  URL) before interpolating into outbound HTML emails — name is free text
  from registration/checkout with no length/char restriction, so it was
  stored-HTML-injectable into every transactional email.
- Fixed api/redeem-gift-card.php referencing a nonexistent `balance` column
  on gift_cards (actual column is current_balance) — gift card redemption
  was completely broken, always returning "no remaining balance".
- Fixed api/submit-review.php inserting into nonexistent `content`/`status`
  columns on reviews (actual columns are `comment`/`is_approved`) — review
  submission was crashing on every request.
- Hardened .htaccess: block /db/*, /.git/*, and *.sql. Live site currently
  serves db/schema.sql and the full .git directory (including .git/config,
  which contains a GitHub PAT with push access) over HTTP — the existing
  config/includes RedirectMatch rules are also not being enforced live,
  see report for details; this needs a server-level fix too.
- Cleaned up README's leftover install instructions pointing at a deleted
  create-admin.php with a documented default password.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.htaccess

@@ -16,6 +16,30 @@ RewriteRule ^ %1 [L,R=301]
 RedirectMatch 403 /config/.*$
 RedirectMatch 403 /includes/.*\.php$
 RedirectMatch 403 /install/.*$
+RedirectMatch 403 /db/.*$
+
+# Block .git (full source + commit history, including any credentials ever
+# committed to it) from ever being served. NOTE: on the live server this
+# repo's .git directory lives inside the docroot itself (deployed via
+# `git clone` directly into public_html) and was confirmed reachable over
+# HTTP (.git/config, .git/logs/HEAD, etc. all returned real content) even
+# though the RedirectMatch rules above were also confirmed NOT blocking
+# /config/*.php or /includes/*.php live — meaning .htaccess directives are
+# not being fully honored by the current web server. This needs a server
+# config fix (vhost-level deny, or moving .git outside the docroot) in
+# addition to this file — see review notes.
+RedirectMatch 403 /\.git/.*$
+RedirectMatch 403 /\.git$
+
+<FilesMatch "\.sql$">
+    <IfModule mod_authz_core.c>
+        Require all denied
+    </IfModule>
+    <IfModule !mod_authz_core.c>
+        Order allow,deny
+        Deny from all
+    </IfModule>
+</FilesMatch>
 
 # Set default charset
 AddDefaultCharset UTF-8