feat(terminal): paste clipboard images into SSH sessions as a file path (#630)

[kipavy]

Takes over #630 by @ghecko with the changes from its review. His setting, translations, shared paste helper and terminal hookup are kept, and the squash commit credits him as co-author.

What

When the clipboard holds an image and no text, pasting into an SSH session uploads the image to the host and types its path, followed by a space. Paste works with Ctrl+V, Ctrl+Shift+V, right-click, and now also Cmd+V on macOS. Tools on the host such as Claude Code then attach it.

• Uploads that take more than 250 ms show a bottom-right progress toast with a thumbnail, host, size, dimensions, percent and Cancel.
• On success the toast shows the file and offers "Delete from host". The very first paste says instead that the image is on the host for 7 days, readable only by you, and links to the setting.
• On failure the toast names the host and the classified error (e.g. "prod-api: Permission denied") and offers Retry, which re-sends the same image.
• Pastes queue per session, so two quick pastes type their paths in order.
• Nothing uploads when the session doesn't accept input (e.g. control handed to a guest). While input is broadcast to several panes, paste refuses with a toast.
• The setting description now notes that the path points to the first host if you SSH further.

How

• paste_image_read reads the clipboard in Rust, encodes PNG with png (already in the tree) and returns the size, dimensions and a small thumbnail. It keeps the last four images by id for Retry. No webview clipboard-image permission is needed.
• paste_image_upload opens SFTP on the session's existing connection. It ensures ~/.cache/voltius/pastes exists and is owner-only, and prunes pastes older than 7 days. It writes <name>.part in 64 KB chunks with sftp-progress-<id> events and a cancel token, then renames to the final name. A failed or cancelled write deletes the .part file.
• Progress toasts can now carry a thumbnail, a detail line and an action once finished. A finished one keeps its own duration instead of the fixed 2 s.
• sessionAcceptsInput is shared with writeToSession.
• Compared with feat(terminal): paste clipboard images into SSH sessions as a file path #630, this drops sftp_write_bytes and its header plumbing, the FileBackend::write_file(&[u8]) change, the canvas re-encode, and clipboard-manager:allow-read-image.

Testing

• Rust: commands::sftp::paste tests cover the file name, prune rules, thumbnail aspect and Retry memory. cargo clippy --workspace --all-targets -- -D warnings is clean.
• Frontend: terminalImagePaste.test.ts covers no image, toggle off, no input, broadcast, fast and slow uploads, progress, Cancel, first use, failure with Retry, and queue order. terminalClipboard.test.ts adds macOS Cmd+V. tsc is clean. The full vitest sweep (802 files) passed apart from two known 5 s timeouts under load, which pass when run on their own.
• In the headless app against an OpenSSH host, with real clipboard images:
  • First and later pastes.
  • Delete from host (file gone).
  • A 3840×2360 screenshot over a link throttled to 2 Mbit/s, with live progress and Cancel at 32% (no file or .part left).
  • Upload failure with Retry.
  • Two pastes in a row.
• Known limit: the X11 clipboard read gives up after about 4 s on huge transfers (a 13 MB noise PNG). Real screenshots, including 4K at 2 MB, read fine.