Feature/delta team/epic r04 (#7)

## Summary

epic r04 - begin refactor

## Change Type

- [x] Cowork feature
- [ ] Bug fix
- [ ] Core AI contribution
- [ ] Test / hardening
- [ ] Performance
- [ ] Documentation

## Related Work

Cowork Task:

Core Repo: http://34.143.229.138/gitea-admin/fsg-ai-core-assets

Core AI Issue:

Core Task:

Related PR:

## Scope

What is intentionally included?

What is intentionally NOT included?

## Validation

- [ ] Unit tests
- [ ] Integration tests
- [ ] Manual verification
- [ ] Regression check

Commands / evidence:

## Security Impact

Permission / credential / network / customer data impact:

## Compatibility

- [ ] No breaking change
- [ ] Breaking change documented

## Reviewer Notes

Anything Cowork reviewers should pay attention to.

---------

Co-authored-by: Anh Tran Nguyen Minh <anhtnm1@fpt.com>
Co-authored-by: Huong Le Thi Thien <huongltt35@fpt.com>
Co-authored-by: Nam Pham Dinh Thanh <nampdt@fpt.com>
Co-authored-by: Vu Dam Tuan <vudt15@fpt.com>
Co-authored-by: Hiep Ha Van <hiephv3@fpt.com>
Co-authored-by: Lam Hoang Van <lamhv7@fpt.com>
Reviewed-on: #7
Co-authored-by: Duy Le Huu <duylh19@fpt.com>

application/workspaces/file_preview_helpers.py

@@ -0,0 +1,62 @@
+"""Pure helpers for previewing a file (R08-T12, moved out of
+``ui/folder_tab.py`` — that file's module-level functions
+``_read_text``/``_is_probably_text``/``_pptx_available``, lines 1519-1533 and
+1565-1587 of the original 1587-line file). No Qt, no widget state — the
+"is this file text? is pptx editing available?" questions the preview
+manager asks before it decides how to render something.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+_PPTX_READY = None   # cached: pptx-editing library available (after auto-install)
+
+
+def pptx_available() -> bool:
+    """True when python-pptx is importable. If it's MISSING, auto-download &
+    install it (via deps.ensure_module) so pptx editing 'just works' — cached
+    so the (one-time) install is attempted only once."""
+    global _PPTX_READY
+    if _PPTX_READY is None:
+        try:
+            from cowork_local.core.deps import ensure_module
+
+            _PPTX_READY = ensure_module("pptx", "python-pptx") is not None
+        except Exception:  # noqa: BLE001
+            _PPTX_READY = False
+    return _PPTX_READY
+
+
+def read_text(path: str) -> str:
+    """Đọc tệp dạng văn bản, thay ký tự hỏng thay vì ném lỗi; không đọc được thì
+    trả về chuỗi rỗng.
+    """
+    try:
+        return Path(path).read_text(encoding="utf-8", errors="replace")
+    except OSError as exc:
+        return f"[could not read file: {exc}]"
+
+
+def is_probably_text(path: str) -> bool:
+    """Đoán tệp này có phải văn bản không, bằng cách tìm byte NUL trong phần đầu.
+
+    Đoán sai theo hướng "là văn bản" sẽ hiện một màn hình ký tự rác, nên phép
+    thử cố tình bảo thủ.
+    """
+    try:
+        with open(path, "rb") as f:
+            chunk = f.read(4096)
+    except OSError:
+        return False
+    if b"\x00" in chunk:
+        return False
+    try:
+        chunk.decode("utf-8")
+        return True
+    except UnicodeDecodeError:
+        # Latin-ish text still edits fine via errors="replace"; only reject on
+        # a hard binary signal (NUL above), so most source files pass.
+        return True
+
+
+__all__ = ["pptx_available", "read_text", "is_probably_text"]