app-shell sftp: split commands/sftp.rs into one file per command group #844

Goal

Each group of file-browser commands lives in its own file, so a change to, say, chmod no longer means loading a 3,400-line file.

Context

crates/app-shell/src/commands/sftp.rs has 3,449 lines (~2,660 production lines before the tests at :2666, plus 4,500 lines of tests in commands/sftp/*_tests.rs), 32 Tauri commands and 99 functions, each command with *_impl/*_reporting/*_recursive variants. Sub-modules already exist: commands/sftp/transfer.rs, commands/sftp/folder_upload.rs, and test modules revocation_tests.rs, size_limit_tests.rs, local_path_tests.rs (declared at :51–72); inline test modules rename_overwrite_tests (:2667), sftp_mutation_tests (:2744), edit_session_tests (:3348).

Command groups (line numbers on develop):

• list/stat/exists: sftp_list :75, sftp_start_directory :97, session_is_root :112, list_impl :123, sftp_exists :1254, exists_impl :1271, sftp_stat :1294;
• download: download_one_file :162, sftp_download :194, download_impl :214, pick_folder_dialog :304/:324, default_downloads_dir :337, download_recursive :360, download_entry_to :433, sftp_download_default :486, sftp_download_dir :522, sftp_cancel_transfer :560;
• upload and clipboard: sftp_upload :579, pick_upload_files :654, claim_dropped_paths :712, clipboard_has_files :739, clipboard_write_text :755, ClipboardPasteDialogTexts :769, paste_dialog_message :781, claim_clipboard_files :840, upload_impl :906;
• delete: walk_dirs_and_count_files :940, EntryRows :965, sftp_delete_preview :1059, delete_preview_impl :1076, sftp_delete :1114, delete_impl* :1136–1186, delete_recursive :1187;
• permissions: sftp_chmod :1323, chmod_impl* :1357–1423, recursive_entry_mode :1424, chmod_recursive :1441, sftp_chown :1513, chown_impl*, resolve_chown_ids :1574, chown_target :1656, chown_recursive :1677;
• rename/mkdir/create: rename_replacing :1741, sftp_rename :1771, sftp_mkdir :1813, mkdir_impl :1830, create_file_impl :1848, sftp_create_file :1875;
• copy content: too_large_for_clipboard :1910, sftp_read_text :1932, read_text_impl :1949, CopyContentProgress/CopyContentJobs :2010–2057, sftp_copy_content :2060, sftp_cancel_copy_content :2101, copy_content_* :2109–2186, sftp_copy_file :2189, read_local_text_preview :2261;
• edit session: sftp_open_for_editing :2378, open_for_editing_impl :2415, local_file_mtime :2487, in_app_edit_copy :2532, read_edit_copy_text :2593, write_edit_copy_text :2610, close_edit_session* :2630–2665.

Requirements

• commands/sftp.rs becomes commands/sftp/mod.rs (shared imports, re-exports of all #[tauri::command] functions so lib.rs's generate_handler! paths keep working) plus commands/sftp/{list.rs, download.rs, upload_clipboard.rs, delete.rs, permissions.rs, rename_mkdir.rs, copy_content.rs, edit_session.rs}.
• Inline test modules move next to their code (rename_overwrite_tests → with rename_mkdir.rs, edit_session_tests → with edit_session.rs, sftp_mutation_tests → split by the function they test, or kept as sftp/tests_mutation.rs).
• No logic change; no moving of code to app-logic in this issue (separate issue).

Acceptance Criteria

• crates/app-shell/src/commands/sftp.rs no longer exists; the listed files exist under commands/sftp/.
• Every file under crates/app-shell/src/commands/sftp/ has a production part below 800 lines.
• npm run check-commands (frontend) and cargo build --workspace pass without changing any command name.
• Test count per binary unchanged, all pass.
• Gate green.

Technical Notes

• The file browser can run on an elevated (sudo) SFTP channel. BrowserChannel, BrowserAccess and ElevatedSftpRegistry stay private to app-shell exactly as today (docs/architecture.md, "Eine Ausnahme … der erhöhte SFTP-Kanal"); do not widen any visibility while moving.
• The recursive delete uses lstat to never follow a symlink into another tree (Spec 0054); keep the code byte-identical apart from module paths.
• Restructuring only: behaviour, error codes, emitted events, log fields and messages stay identical. Fix nothing on the way; a bug you find goes into its own issue (mention it in the PR).
• Tests: cargo test --workspace -- --list before and after; the number of tests per test binary does not decrease, and the same tests pass. Tests may move with the code they test (<module>/tests.rs or <module>/tests_<topic>.rs; no new *_tests.rs names).
• Public items used from outside the crate keep their paths (re-export if an item moves); check with grep -rn over the workspace.
• New code comments in English; existing German comments may move unchanged.
• Update docs/architecture.md (module table "Ausnahmen von der Regel (A4)" and the list of app-shell modules) when a module moves between app-shell and app-logic.
• Gate: cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace, cargo build --workspace (the last one catches production calls into test-support-only items).

Spec impact: none

Related Work

• Next: File browser backend: move recursive delete/chmod/chown, mode-bit and owner logic from app-shell to app-logic::sftp_ops #849