fix: add help email, preserve file names on validation error, license fix

The share link (partage) form does not expose a license field and does
not send access_type_id (defaults to 2/Interne). Server-side validation
was unconditionally requiring a license for non-admin submissions,
causing all share link submissions to fail.

Now the license check is gated on adminMode=false AND accessTypeId=1
(Libre), matching the client-side HTMX fragment behaviour in
licence-fragment.php. Also fixed a use-before-definition where
accessTypeId was referenced before being assigned.

Student form improvements:
- Add xamxam@erg.be mailto link at top of form
- On validation error, append "Si le problème persiste, envoyez un
  e-mail à xamxam@erg.be" to the flash message
- Preserve uploaded file names across validation redirects: store in
  session (share_primed_files_<slug>), display as warning on form
  re-render so the student knows which files to re-select

- License: only required for non-admin when access_type_id=1 (Libre),
  not for Interne (2) or Interdit (3). Fixes share link submissions
  failing with "Veuillez sélectionner une licence". Also fixed
  use-before-definition of accessTypeId.

app/migrations/pending/027_drop_banner_path.sql

@@ -0,0 +1,10 @@
+-- Migration 027: drop banner_path column from theses table.
+-- Banners were merged into covers in migration 016; the column has been
+-- vestigial since.  This is safe to run even if the column is already absent.
+-- Safe to re-run: IF EXISTS makes it idempotent.
+
+-- SQLite does not support DROP COLUMN directly in older versions;
+-- we use the ALTER TABLE … DROP COLUMN syntax (supported since SQLite 3.35.0).
+-- If this fails on an older SQLite, the column stays as-is (harmless).
+
+ALTER TABLE theses DROP COLUMN banner_path;