feat: add document conversion support (Markdown, HTML, Plain Text)

- Add marked for Markdown to HTML conversion with GFM support
- Add turndown for HTML to Markdown conversion
- Add DOMPurify for HTML sanitization (security)
- Support Markdown ↔ HTML ↔ Plain Text conversions
- Add styled HTML output with responsive design
- Use client-side only DOMPurify to fix SSR issues

Supported conversions:
- Markdown → HTML (with code syntax, tables, blockquotes)
- HTML → Markdown (clean formatting preservation)
- Markdown/HTML → Plain Text (strip formatting)
- Plain Text → HTML/Markdown (basic formatting)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

pnpm-lock.yaml

@@ -20,12 +20,18 @@ importers:
       clsx:
         specifier: ^2.1.1
         version: 2.1.1
+      dompurify:
+        specifier: ^3.2.2
+        version: 3.3.0
       fuse.js:
         specifier: ^7.1.0
         version: 7.1.0
       lucide-react:
         specifier: ^0.553.0
         version: 0.553.0(react@19.2.0)
+      marked:
+        specifier: ^15.0.4
+        version: 15.0.12
       next:
         specifier: ^16.0.0
         version: 16.0.3(@babel/core@7.28.5)(react-dom@19.2.0(react@19.2.0))(react@19.2.0)
@@ -38,6 +44,9 @@ importers:
       tailwind-merge:
         specifier: ^3.3.1
         version: 3.4.0
+      turndown:
+        specifier: ^7.2.0
+        version: 7.2.2
     devDependencies:
       '@tailwindcss/postcss':
         specifier: ^4.1.17
@@ -51,6 +60,9 @@ importers:
       '@types/react-dom':
         specifier: ^19
         version: 19.2.3(@types/react@19.2.5)
+      '@types/turndown':
+        specifier: ^5.0.5
+        version: 5.0.6
       eslint:
         specifier: ^9
         version: 9.39.1(jiti@2.6.1)
@@ -368,6 +380,9 @@ packages:
   '@jridgewell/trace-mapping@0.3.31':
     resolution: {integrity: sha512-zzNR+SdQSDJzc8joaeP8QQoCQr8NuYx2dIIytl1QeBEZHJ9uW6hebsrYgbz8hJwUQao3TWCMtmfV8Nu1twOLAw==}
 
+  '@mixmark-io/domino@2.2.0':
+    resolution: {integrity: sha512-Y28PR25bHXUg88kCV7nivXrP2Nj2RueZ3/l/jdx6J9f8J4nsEGcgX0Qe6lt7Pa+J79+kPiJU3LguR6O/6zrLOw==}
+
   '@napi-rs/wasm-runtime@0.2.12':
     resolution: {integrity: sha512-ZVWUcfwY4E/yPitQJl481FjFo3K22D6qF0DuFH6Y/nbnE11GY5uguDxZMGXPQ8WQ0128MXQD7TnfHyK4oWoIJQ==}
 
@@ -558,6 +573,12 @@ packages:
   '@types/react@19.2.5':
     resolution: {integrity: sha512-keKxkZMqnDicuvFoJbzrhbtdLSPhj/rZThDlKWCDbgXmUg0rEUFtRssDXKYmtXluZlIqiC5VqkCgRwzuyLHKHw==}
 
+  '@types/trusted-types@2.0.7':
+    resolution: {integrity: sha512-ScaPdn1dQczgbl0QFTeTOmVHFULt394XJgOQNoyVhZ6r2vLnMLJfBPd53SB52T/3G36VI1/g2MZaX0cwDuXsfw==}
+
+  '@types/turndown@5.0.6':
+    resolution: {integrity: sha512-ru00MoyeeouE5BX4gRL+6m/BsDfbRayOskWqUvh7CLGW+UXxHQItqALa38kKnOiZPqJrtzJUgAC2+F0rL1S4Pg==}
+
   '@typescript-eslint/eslint-plugin@8.46.4':
     resolution: {integrity: sha512-R48VhmTJqplNyDxCyqqVkFSZIx1qX6PzwqgcXn1olLrzxcSBDlOsbtcnQuQhNtnNiJ4Xe5gREI1foajYaYU2Vg==}
     engines: {node: ^18.18.0 || ^20.9.0 || >=21.1.0}
@@ -910,6 +931,9 @@ packages:
     resolution: {integrity: sha512-35mSku4ZXK0vfCuHEDAwt55dg2jNajHZ1odvF+8SSr82EsZY4QmXfuWso8oEd8zRhVObSN18aM0CjSdoBX7zIw==}
     engines: {node: '>=0.10.0'}
 
+  dompurify@3.3.0:
+    resolution: {integrity: sha512-r+f6MYR1gGN1eJv0TVQbhA7if/U7P87cdPl3HN5rikqaBSBxLiCb/b9O+2eG0cxz0ghyU+mU1QkbsOwERMYlWQ==}
+
   dunder-proto@1.0.1:
     resolution: {integrity: sha512-KIN/nDJBQRcXw0MLVhZE9iQHmG68qAVIBg9CqmUYjmQIhgij9U5MFvrqkUL5FbtyyzZuOeOt0zdeRe4UY7ct+A==}
     engines: {node: '>= 0.4'}
@@ -1509,6 +1533,11 @@ packages:
   magic-string@0.30.21:
     resolution: {integrity: sha512-vd2F4YUyEXKGcLHoq+TEyCjxueSeHnFxyyjNp80yg0XV4vUhnDer/lvvlqM/arB5bXQN5K2/3oinyCRyx8T2CQ==}
 
+  marked@15.0.12:
+    resolution: {integrity: sha512-8dD6FusOQSrpv9Z1rdNMdlSgQOIP880DHqnohobOmYLElGEqAL/JvxvuxZO16r4HtjTlfPRDC1hbvxC9dPN2nA==}
+    engines: {node: '>= 18'}
+    hasBin: true
+
   math-intrinsics@1.1.0:
     resolution: {integrity: sha512-/IXtbwEk5HTPyEwyKX6hGkYXxM9nbj64B+ilVJnC/R6B0pH5G4V3b0pVbL7DBj4tkhBAppbQUlf6F6Xl9LHu1g==}
     engines: {node: '>= 0.4'}
@@ -1871,6 +1900,9 @@ packages:
   tslib@2.8.1:
     resolution: {integrity: sha512-oJFu94HQb+KVduSUQL7wnpmqnfmLsOA/nAh6b6EH0wCEoK0/mPeXU6c3wKDV83MkOuHPRHtSXKKU99IBazS/2w==}
 
+  turndown@7.2.2:
+    resolution: {integrity: sha512-1F7db8BiExOKxjSMU2b7if62D/XOyQyZbPKq/nUwopfgnHlqXHqQ0lvfUTeUIr1lZJzOPFn43dODyMSIfvWRKQ==}
+
   type-check@0.4.0:
     resolution: {integrity: sha512-XleUoc9uwGXqjWwXaUTZAmzMcFZ5858QA2vvx1Ur5xIcixXIP+8LnFDgRplU30us6teqdlskFfu+ae4K79Ooew==}
     engines: {node: '>= 0.8.0'}
@@ -2266,6 +2298,8 @@ snapshots:
       '@jridgewell/resolve-uri': 3.1.2
       '@jridgewell/sourcemap-codec': 1.5.5
 
+  '@mixmark-io/domino@2.2.0': {}
+
   '@napi-rs/wasm-runtime@0.2.12':
     dependencies:
       '@emnapi/core': 1.7.1
@@ -2415,6 +2449,11 @@ snapshots:
     dependencies:
       csstype: 3.2.2
 
+  '@types/trusted-types@2.0.7':
+    optional: true
+
+  '@types/turndown@5.0.6': {}
+
   '@typescript-eslint/eslint-plugin@8.46.4(@typescript-eslint/parser@8.46.4(eslint@9.39.1(jiti@2.6.1))(typescript@5.9.3))(eslint@9.39.1(jiti@2.6.1))(typescript@5.9.3)':
     dependencies:
       '@eslint-community/regexpp': 4.12.2
@@ -2788,6 +2827,10 @@ snapshots:
     dependencies:
       esutils: 2.0.3
 
+  dompurify@3.3.0:
+    optionalDependencies:
+      '@types/trusted-types': 2.0.7
+
   dunder-proto@1.0.1:
     dependencies:
       call-bind-apply-helpers: 1.0.2
@@ -3520,6 +3563,8 @@ snapshots:
     dependencies:
       '@jridgewell/sourcemap-codec': 1.5.5
 
+  marked@15.0.12: {}
+
   math-intrinsics@1.1.0: {}
 
   merge2@1.4.1: {}
@@ -3947,6 +3992,10 @@ snapshots:
 
   tslib@2.8.1: {}
 
+  turndown@7.2.2:
+    dependencies:
+      '@mixmark-io/domino': 2.2.0
+
   type-check@0.4.0:
     dependencies:
       prelude-ls: 1.2.1