feat: implement English Style Converter

- SvelteKit project scaffolded with TypeScript
- Type definitions for Style, StyleCategory, ConversionRequest, ConversionResponse, LLMConfig
- Style definitions with 6 categories and 25 sub-styles
- Intensity mapping (1-5) with prompt modifier placeholders
- LLM client using OpenAI-compatible API (Ollama default)
- POST /api/convert endpoint with input validation
- Animated loading modal with per-letter animations
- Main page UI with category/style selectors, intensity slider
- Copy to clipboard, collapsible prompt display
- Vitest tests for styles, LLM prompt building, and API validation
- Environment configuration for LLM settings

src/routes/api/convert/+server.ts

@@ -0,0 +1,62 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { getStyleById, getIntensityConfig } from '$lib/styles';
+import { convertText } from '$lib/llm';
+import type { ConversionRequest, ConversionResponse } from '$lib/types';
+
+export const POST: RequestHandler = async ({ request }) => {
+	let body: ConversionRequest;
+	try {
+		body = await request.json();
+	} catch {
+		return json({ error: 'Invalid JSON body' }, { status: 400 });
+	}
+
+	const { text, styleId, intensity } = body;
+
+	// Validate text
+	if (!text || typeof text !== 'string' || text.trim().length === 0) {
+		return json({ error: 'Text is required and must be non-empty' }, { status: 400 });
+	}
+
+	// Validate styleId
+	if (!styleId || typeof styleId !== 'string') {
+		return json({ error: 'styleId is required' }, { status: 400 });
+	}
+
+	const style = getStyleById(styleId);
+	if (!style) {
+		return json({ error: `Unknown style: ${styleId}` }, { status: 400 });
+	}
+
+	// Validate intensity
+	if (typeof intensity !== 'number' || !Number.isInteger(intensity) || intensity < 1 || intensity > 5) {
+		return json({ error: 'Intensity must be an integer between 1 and 5' }, { status: 400 });
+	}
+
+	const intensityConfig = getIntensityConfig(intensity);
+	if (!intensityConfig) {
+		return json({ error: 'Invalid intensity level' }, { status: 400 });
+	}
+
+	try {
+		const result = convertText(text, style.promptModifier, intensityConfig.instruction);
+
+		// Await the promise
+		const { converted, systemPrompt, userMessage } = await result;
+
+		const response: ConversionResponse = {
+			original: text,
+			converted,
+			styleId,
+			intensity,
+			systemPrompt,
+			userMessage
+		};
+
+		return json(response);
+	} catch (err) {
+		const message = err instanceof Error ? err.message : 'LLM call failed';
+		return json({ error: `Failed to convert text: ${message}` }, { status: 502 });
+	}
+};