feat(pdf): three-level quality (Normal/Compressed/Draft) + download modal

- compilePdfQuality: 'none' | 'ebook' | 'screen' replaces compressPdf: boolean
  throughout the stack (types, context, compiler, RequestParser, CompileManager)
- Compile dropdown now shows Normal / Compressed / Draft under "PDF quality"
  (ebook=150dpi, screen=72dpi via Ghostscript)
- Download button opens a modal with the same three quality options plus
  per-option descriptions and a Ghostscript attribution note; compressed
  downloads are served by a new /download/.../output.pdf/compressed endpoint
  that fetches the last build from clsi and pipes it through GS on demand

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/clsi/app/js/CompileManager.js

@@ -318,8 +318,8 @@ async function doCompile(request, stats, timings) {
     'done compile'
   )
 
-  if (request.compressPdf) {
-    await _compressOutputPdf(compileDir)
+  if (request.compilePdfQuality && request.compilePdfQuality !== 'none') {
+    await _compressOutputPdf(compileDir, request.compilePdfQuality)
   }
 
   const { outputFiles, buildId } = await _saveOutputFiles({
@@ -449,7 +449,8 @@ async function clearExpiredProjects(maxCacheAgeMs) {
   }
 }
 
-async function _compressOutputPdf(compileDir) {
+async function _compressOutputPdf(compileDir, quality) {
+  const preset = quality === 'screen' ? '/screen' : '/ebook'
   const src = Path.join(compileDir, 'output.pdf')
   const tmp = Path.join(compileDir, 'output-gs-tmp.pdf')
   const t0 = Date.now()
@@ -461,7 +462,7 @@ async function _compressOutputPdf(compileDir) {
           '-q', '-dNOPAUSE', '-dBATCH',
           '-sDEVICE=pdfwrite',
           '-dCompatibilityLevel=1.7',
-          '-dPDFSETTINGS=/ebook',
+          `-dPDFSETTINGS=${preset}`,
           `-sOutputFile=${tmp}`,
           src,
         ],

services/clsi/app/js/RequestParser.js

@@ -91,10 +91,10 @@ function parse(body, callback) {
       default: false,
       type: 'boolean',
     })
-    response.compressPdf = _parseAttribute(
-      'compressPdf',
-      compile.options.compressPdf,
-      { default: false, type: 'boolean' }
+    response.compilePdfQuality = _parseAttribute(
+      'compilePdfQuality',
+      compile.options.compilePdfQuality,
+      { default: 'none', type: 'string' }
     )
     response.stopOnFirstError = _parseAttribute(
       'stopOnFirstError',