Synopsis : normalisation SÛRE des text ajouté par formulaire

- fins de ligne \r\n/\r → \n,
- blocs de lignes vides → \n\n, è
- espaces/tabulations/NBSP → espace unique,
- trim.
Les retours à la ligne simples intra-paragraphe sont LAISSÉS INTACTS :
une coupure de ligne peut être une vraie frontière de mot (→ espace)
ou une coupure du mot ("dyna\nmiques" → "dynamiques"),
indistinguables sans dictionnaire — automatiser corromprait l'entrée
("poursuivantsur").

Migration batch 045 (backfill des données existantes) RETIRÉE : le
nettoyage automatique fiable des synopsis existants est impossible.
Chaque modification est testée (277 tests PHPUnit).

app/src/Controllers/ThesisCreateController.php

@@ -609,12 +609,39 @@ class ThesisCreateController
     // ── Private: input helpers ────────────────────────────────────────────────
 
     /**
-     * Trim and strip HTML tags from a string value.
-     * htmlspecialchars is applied at render time, not here.
+     * Normalise a submitted string value.
+     *
+     * Only the UNAMBIGUOUS cleanups are applied here. We deliberately do NOT
+     * auto-collapse or rejoin single newlines inside a paragraph: a line break
+     * can be either a real word boundary (should become a space) or a word split
+     * across the line ("dyna\nmiques", should be rejoined to "dynamiques"),
+     * and these are indistinguishable without a dictionary. Trying to guess
+     * corrupts valid input (e.g. "se poursuivant\nsur" → "se poursuivantsur"),
+     * so such breaks are left untouched and must be fixed by the author.
+     *
+     * Responsibilities:
+     *  - strip HTML tags (htmlspecialchars is applied at render time, not here)
+     *  - normalise all line endings to \n (handles \r\n and bare \r)
+     *  - collapse runs of horizontal whitespace (spaces, tabs, NBSP) to one space
+     *  - collapse multiple blank lines into a single paragraph separator (\n\n)
+     *  - trim leading/trailing whitespace and blank lines
      */
     private function sanitiseString(string $input): string
     {
-        return strip_tags(trim($input));
+        $input = strip_tags($input);
+
+        // Normalise all line endings to \n (handles \r\n and bare \r).
+        $input = preg_replace('/\r\n|\r|\n/', "\n", $input);
+
+        // Collapse multiple blank lines into a single paragraph separator.
+        $input = preg_replace("/\n{2,}/u", "\n\n", $input);
+
+        // Collapse runs of horizontal whitespace (spaces, tabs, NBSP gaps, …)
+        // into a single space. Single newlines inside a paragraph are NOT
+        // touched, for the reason explained above.
+        $input = preg_replace('/[ \t\x0B\x00\xC2\xA0]+/u', ' ', $input);
+
+        return trim($input, " \t\n\r\0\x0B\xC2\xA0");
     }
 
     /**