fix delete failed

[SuperCoolPencil]

• fix: handle missing downloads gracefully by ignoring not-found errors during deletion
• fix: make cancellation idempotent and add resilience for download deletion in TUI
  Closes Cannot delete a non-downloaded file #373

Greptile Summary

This PR fixes a delete-failure bug by making Cancel idempotent (returning nil for missing downloads) and replacing brittle string-matching in the TUI with a proper errors.Is(err, types.ErrNotFound) guard. It also consolidates scattered fmt.Errorf literals into a shared sentinel errors package in internal/engine/types/errors.go.

Confidence Score: 5/5

Safe to merge; only a P2 style inconsistency found.

All findings are P2. The core fix (idempotent Cancel + sentinel error guard in TUI) is correct and well-tested. The only gap is one missed fmt.Errorf in ResumeBatch that was not converted alongside the rest of the refactoring.

internal/processing/pause_resume.go — ResumeBatch cold path at line 217 still uses fmt.Errorf instead of types.ErrNotFound.

Important Files Changed

Filename	Overview
internal/engine/types/errors.go	Introduces sentinel errors to replace scattered fmt.Errorf calls; clean and well-structured.
internal/processing/pause_resume.go	Cancel made idempotent with clear comment explaining the rationale; one fmt.Errorf was missed in ResumeBatch cold path.
internal/tui/update_dashboard.go	Delete handler now uses errors.Is(err, types.ErrNotFound) instead of brittle string matching — the primary TUI fix.
internal/tui/delete_resilience_test.go	New test file covering ErrNotFound, success, and other-error paths for the TUI delete handler; good coverage of the defensive layer.
internal/core/local_service.go	Inline fmt.Errorf calls replaced with sentinel errors; GetStatus now returns types.ErrNotFound for missing downloads.
internal/download/pool.go	UpdateURL error strings replaced with ErrQueuedUpdate and ErrActiveUpdate sentinels.
internal/engine/concurrent/downloader.go	10-redirect guard replaced with ErrMaxRedirects sentinel; no behavioral change.
internal/engine/single/downloader.go	Same ErrMaxRedirects sentinel change as concurrent downloader; consistent.
internal/processing/manager.go	Enqueue/EnqueueWithID nil-func guards replaced with ErrServiceUnavailable sentinel; straightforward.
internal/processing/manager_test.go	Cancel_NotFound test correctly inverted to expect nil error after idempotent Cancel change.

Prompt To Fix All With AI

This is a comment left during a code review.
Path: internal/processing/pause_resume.go
Line: 217

Comment:
**Missed sentinel error in `ResumeBatch` cold path**

This `fmt.Errorf("download not found or completed")` was not converted as part of the sentinel-error refactoring in this PR. Any caller (including the TUI) that tries `errors.Is(err, types.ErrNotFound)` on a `ResumeBatch` result will silently miss this case.

```suggestion
errs[idx] = types.ErrNotFound
```

**Rule Used:** What: Eliminate duplicate logic, functions, or cod... ([source](https://app.greptile.com/review/custom-context?memory=f0a796a9-684f-4dfb-a5cf-8c99c16b63a1))

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (2): Last reviewed commit: "refactor: centralize error handling by d..." | Re-trigger Greptile}

Context used:

• Rule used - What: Eliminate duplicate logic, functions, or cod... (source)