fix(bookslm): 422 + thème + picker zindex

- Bug 1 (422): bookslm.js envoie maintenant {message, conversation_history}
  conformes au backend (au lieu de messages:[...])
- Bug 2 (thème BooksLM): style.css lignes 9015-9061 — suppression de
  tous les fallbacks hex (--bg, --muted, --text, --surface, --accent,
  --border) au profit des variables ObsiGate existantes (--bg-primary,
  --text-primary, --text-secondary, --surface, --surface2, --accent,
  --border). Ajout de color: var(--text-primary) sur .bookslm-panel
- Bug 3 (picker zindex/flex): bookslm-picker-host + ai-picker +
  .bookslm-header button obtiennent flex-shrink:0 + z-index:1 pour
  éviter que les selects compressent ou masquent les boutons
  header (Nouvelle conversation / Exporter / Plein écran / Fermer)
- Bug 4 (theme picker): ai.js — var(--bg-tertiary, transparent) →
  var(--surface), var(--border-color) → var(--border) sur tous les
  selects, menus et createSeparator. Plus de variables inexistantes.

Tests: pytest 504 passed, frontend unit 7/7, validate-imports 30 modules
204 exports, pane-manager 9/9, ruff All checks passed.

frontend/js/bookslm.js

@@ -311,8 +311,16 @@ class BooksLM {
         body: JSON.stringify({
           vault: this._vault,
           directory: this._directory,
-          messages: this._messages.slice(0, -1), // exclude empty assistant
-          context_files: this._contextFiles.map(f => f.path || f),
+          // The backend expects `message` + `conversation_history`,
+          // not a generic `messages` array. Convert before sending.
+          message: this._messages.length
+            ? this._messages[this._messages.length - 1].content
+            : '',
+          conversation_history: this._messages
+            .slice(0, -1)
+            .filter((m) => m.content && m.content.trim())
+            .map((m) => ({ role: m.role, content: m.content })),
+          context_files: this._contextFiles.map((f) => f.path || f),
           provider,
           model,
         }),