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/public/about.php

@@ -83,9 +83,7 @@ function renderEntries(array $entries): string
                             fn($e) => !empty($e),
                         );
                         foreach ($emails as $email): ?>
-                        <a href="mailto:<?= htmlspecialchars(
-                            $email,
-                        ) ?>"><?= htmlspecialchars($email) ?></a>
+                        <a href="<?= EmailObfuscator::mailto($email) ?>"><?= htmlspecialchars($email) ?></a>
                         <?php endforeach;
                         ?>
                     </address>

app/templates/public/tfe.php

@@ -252,9 +252,7 @@
                                 <span class="sr-only">(ouvre dans un nouvel onglet)</span>
                             </a>
                         <?php elseif ($_isEmail): ?>
-                            <a href="mailto:<?= htmlspecialchars(
-                                $_contact,
-                            ) ?>"><?= htmlspecialchars($_contact) ?></a>
+                            <a href="<?= EmailObfuscator::mailto($_contact) ?>"><?= htmlspecialchars($_contact) ?></a>
                         <?php else: ?>
                             <?= htmlspecialchars($_contact) ?>
                         <?php endif;