fix(ai): contexte applicatif de l'assistant, dossier vide et liens fiables (#88, BUG-041, BUG-042)

- BUG-041 : le contexte d'un dossier vide ne renvoie plus 404 ; il degrade
  vers le prompt General enrichi d'un bloc « Dossier vide ».
- BUG-042 : liens de fichiers deterministes (nom -> presse-papiers,
  dossier -> arborescence, chemin -> viewer) et resolution du chemin contre
  l'index du vault (exact -> suffixe -> basename) avant ouverture, fin des
  'File not found'.
- #88 : app_context (documents ouverts, repertoire/vault courants, recherche
  + resultats affiches) et fichiers recemment modifies injectes dans le
  prompt General.

Tests : pytest 963 passed / 6 skipped, ruff 0, mypy 0, frontend 52/52 +
validate-imports 36 modules.

tests/test_bookslm.py

@@ -527,15 +527,17 @@ class TestBooksLMChatEndpoint:
         )
         assert resp.status_code == 404
 
-    def test_chat_empty_directory(self, bookslm_client):
-        """Chat endpoint returns 404 for empty directory."""
+    def test_chat_empty_directory_does_not_block(self, bookslm_client):
+        """Chat on an empty directory must not 404 — it answers without context."""
         token, _ = _login_bookslm(bookslm_client)
         resp = bookslm_client.post(
             "/api/ai/bookslm/chat",
             json={"vault": "TestVault", "directory": "nonexistent", "message": "Hello"},
             headers={"Authorization": f"Bearer {token}"},
         )
-        assert resp.status_code == 404
+        # 200 (stream opened) or 500 (no AI key) — never 404/422.
+        assert resp.status_code in (200, 500), f"unexpected {resp.status_code}: {resp.text[:200]}"
+        assert resp.status_code != 404
 
     def test_chat_request_accepts_provider_and_model(self, bookslm_client):
         """The chat endpoint schema accepts provider + model fields without rejecting.
@@ -641,6 +643,36 @@ class TestGeneralPrompt:
         assert "Beta" in prompt
         assert "obsigate-action" in prompt
 
+    def test_general_prompt_includes_app_context(self):
+        """The General assistant is told what the user is currently doing."""
+        from backend.bookslm import build_general_system_prompt
+
+        prompt = build_general_system_prompt(
+            ["Alpha"],
+            app_context={
+                "vault": "Alpha",
+                "directory": "notes",
+                "current_path": "notes/a.md",
+                "open_documents": [{"vault": "Alpha", "path": "notes/a.md"}],
+                "search_query": "auth",
+                "search_total": 3,
+                "search_results": [{"vault": "Alpha", "path": "notes/auth.md"}],
+            },
+            recent_files=[{"vault": "Alpha", "path": "notes/recent.md"}],
+        )
+
+        assert "Contexte applicatif actuel" in prompt
+        assert "notes/a.md" in prompt
+        assert "notes/recent.md" in prompt
+        assert "auth" in prompt
+
+    def test_general_prompt_without_app_context_is_unchanged(self):
+        """No context → no app-context block (backwards compatible)."""
+        from backend.bookslm import build_general_system_prompt
+
+        prompt = build_general_system_prompt(["Alpha"])
+        assert "Contexte applicatif actuel" not in prompt
+
     def test_documents_prompt_scope(self):
         from backend.bookslm import build_system_prompt