feat: Add Export as .txt file option

[HuiNeng6]

Add TXT export option alongside PDF and DOCX.

Backend: Added txt format handling with text/plain content type
Frontend: Added Export TXT button and updated handleExport

Fixes: #5

[qodo-code-review]

Review Summary by Qodo

Add TXT export format alongside PDF and DOCX

✨ Enhancement

Walkthroughs

Description

• Add TXT export format support to backend with text/plain content type
• Update frontend handleExport function to accept txt format parameter
• Add Export TXT button in header with consistent styling
• Update error message to reflect all supported export formats

Diagram

flowchart LR
  User["User clicks Export TXT"] -->|txt format| Frontend["Frontend handleExport"]
  Frontend -->|POST /api/export| Backend["Backend exportController"]
  Backend -->|Buffer.from utf-8| TxtHandler["TXT Handler"]
  TxtHandler -->|text/plain| Download["Download .txt file"]

Loading

File Changes

1. backend/src/controllers/exportController.ts ✨ Enhancement +5/-1

Add TXT export format handling to backend

• Added txt format handling branch with Buffer.from for UTF-8 encoding
• Set content type to text/plain for txt exports
• Updated error message to include txt as valid export format

backend/src/controllers/exportController.ts

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2. frontend/src/App.tsx ✨ Enhancement +9/-2

Add TXT export button and update export handler

• Updated handleExport function signature to accept txt format type
• Added Export TXT button with FileText icon and consistent styling
• Added border-r class to DOCX button for visual consistency
• TXT button calls handleExport with txt format parameter

frontend/src/App.tsx

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[qodo-code-review]

Code Review by Qodo

🐞 Bugs (1) 📘 Rule violations (0) 📎 Requirement gaps (0) 📐 Spec deviations (0)

1. TXT charset not specified 🐞 Bug ✓ Correctness

Description

handleExport encodes TXT as UTF-8 bytes but responds with Content-Type: text/plain (no charset),
and the frontend re-wraps the response blob without preserving the response MIME type. This can
cause clients to misinterpret text encoding (especially for non-ASCII) and results in a Blob with an
empty/incorrect type.

Code

backend/src/controllers/exportController.ts[R24-27]

+        } else if (format === 'txt') {
+            buffer = Buffer.from(text, 'utf-8');
+            contentType = 'text/plain';
+            extension = 'txt';

Evidence

The backend explicitly creates a UTF-8 buffer but does not advertise UTF-8 in the Content-Type
header; the frontend requests a blob but then creates a new Blob without a type, losing the
server’s MIME metadata.

backend/src/controllers/exportController.ts[24-36]
frontend/src/App.tsx[180-209]

Agent prompt

The issue below was found during a code review. Follow the provided context and guidance below and implement a solution

### Issue description
TXT export encodes bytes as UTF-8 but the response does not specify a charset, and the frontend discards the response MIME type when creating the download Blob.

### Issue Context
- Backend: `Buffer.from(text, 'utf-8')` is used for TXT, but `Content-Type` is set to `text/plain` without `charset=utf-8`.
- Frontend: axios uses `responseType: 'blob'`, but code constructs `new Blob([response.data])` without passing a `type`, which drops the server-provided content type.

### Fix Focus Areas
- backend/src/controllers/exportController.ts[24-36]
- frontend/src/App.tsx[191-209]

### Suggested changes
1. **Backend**: set `contentType = 'text/plain; charset=utf-8'` for TXT.
2. **Frontend**: avoid re-wrapping the Blob (use `URL.createObjectURL(response.data)`), or create the Blob with an explicit `type` from `response.headers['content-type']` (fallback to `text/plain; charset=utf-8`).

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