feat: obfuscate all email addresses and mailto links as HTML entities

Added EmailObfuscator class (src/EmailObfuscator.php) that converts
email addresses to HTML decimal entities (e.g. foo@...)
so browsers render them correctly but bots and scrapers see gibberish.

Methods:
- email($addr): obfuscate for display in HTML content
- mailto($addr): return obfuscated mailto: href
- obfuscateHtml($html): post-process rendered HTML to obfuscate all
  mailto: links (used after Parsedown/Markdown rendering)

Applied to:
- partage/index.php: mailto link at top + error scenarios via _flash_contact
  flag rendered in form.php (outside htmlspecialchars to avoid double-escape)
- admin/acces.php: request email mailto links
- admin/file-access.php: request email mailto links
- public/about.php: contact email mailto links
- public/tfe.php: author contact mailto links
- AboutController: Parsedown output post-processing
- LicenceController: Parsedown output post-processing
- Dispatcher::render(): require_once EmailObfuscator for all public views

Also fixed _flash_contact session flag in form.php partial to show
contact email line on share link validation errors (separate from
flash_error/warning to bypass htmlspecialchars double-escaping).

app/templates/admin/file-access.php

@@ -59,9 +59,7 @@
                         <div class="access-req-card__info">
                             <div>
                                 <strong>Email :</strong>
-                                <a href="mailto:<?= htmlspecialchars($req['email']) ?>">
-                                    <?= htmlspecialchars($req['email']) ?>
-                                </a>
+                                <a href="<?= EmailObfuscator::mailto($req['email']) ?>"><?= htmlspecialchars($req['email']) ?></a>
                             </div>
                             <div>
                                 <strong>Date :</strong>