feat: équations mathématiques (KaTeX) — bloc, inline, barre visuelle, IA

- Bloc équation avec barre visuelle 20 symboles (fractions, intégrales, grec, etc.)
- Math en ligne : tape $x^2$ → rendu KaTeX inline automatique
- Math en bloc : tape $$E=mc^2$$ → converti en bloc
- Génération IA : décrit l'équation en langage naturel → LaTeX
- Service math-from-text + endpoint /api/ai/math-from-text
- CSS KaTeX importé
- i18n FR/EN complet

memento-note/lib/ai/services/math-from-text.service.ts

@@ -0,0 +1,44 @@
+import { getChatProvider } from '../factory'
+import { getSystemConfig } from '@/lib/config'
+
+export class MathFromTextService {
+  async generate(description: string): Promise<string> {
+    if (!description?.trim()) {
+      throw new Error('Description is required')
+    }
+
+    const systemPrompt = `You are a mathematician and LaTeX expert.
+Convert the user's natural language description into a single valid LaTeX equation.
+
+CRITICAL RULES:
+- Output ONLY the raw LaTeX code — no preamble, no explanation, no markdown fence, no \\[ or \\( delimiters.
+- Use standard amsmath/amssymb syntax.
+- Never output \\documentclass, \\begin{document}, or $$ delimiters.
+
+Examples:
+- "the quadratic formula" -> x = \\frac{-b \\pm \\sqrt{b^2 - 4ac}}{2a}
+- "Pythagorean theorem" -> a^2 + b^2 = c^2
+- "Euler's identity" -> e^{i\\pi} + 1 = 0
+- "la dérivée de x au carré" -> \\frac{d}{dx} x^2 = 2x`
+
+    const userPrompt = `Convert this to LaTeX:\n\n${description}`
+
+    const config = await getSystemConfig()
+    const provider = getChatProvider(config)
+    const fullPrompt = `${systemPrompt}\n\n${userPrompt}`
+    const raw = await provider.generateText(fullPrompt)
+    return this.extractLatex(raw)
+  }
+
+  private extractLatex(response: string): string {
+    const codeBlock = response.match(/```(?:latex|tex|math)?\n([\s\S]+?)\n```/)
+    if (codeBlock) return codeBlock[1].trim()
+    return response
+      .replace(/^\\\[|\\\]$/g, '')
+      .replace(/^\\\(|\\\)$/g, '')
+      .replace(/^\$\$|\$\$$/g, '')
+      .trim()
+  }
+}
+
+export const mathFromTextService = new MathFromTextService()