Preferences: add a GUI control for the sparse part-file setting (/eMule/CreateSparseFiles) #653

/eMule/CreateSparseFiles is a functional, EC-wired preference that has no control in either amule or amuleGUI. Users can only change it by hand-editing amule.conf. The amuleapi Web UI already exposes it as files.create_normal; the desktop should offer the same checkbox.

Current state

• The pref is registered as INI-only: Preferences.cpp:1674 pushes it onto s_MiscList instead of binding it to a widget with NewCfgItem(<IDC_*>, …) the way AllocateFullFile does at Preferences.cpp:1378.
• No IDC_* id exists, no checkbox in muuli_wdr.{h,cpp}, no handling in PrefsUnifiedDlg.cpp, and no translatable _() string — so it is absent from the message catalogs too.
• amuleGUI shares PrefsUnifiedDlg, so it is missing in remote mode as well.

Why it matters

The setting has a real effect on how part files are created (PartFile.cpp:360-378) and it is not covered by the existing "Preallocate disk space for new files" checkbox. The three combinations behave differently:

Turning sparse files off matters on filesystems where sparse allocation is unsupported, slow, or interacts badly with backup/dedup tooling, and it makes free-space accounting honest while a download is in progress.

Proposed change

Add a checkbox on the Files preferences page, next to "Preallocate disk space for new files" (muuli_wdr.cpp:1593), bound the same way:

• new id in muuli_wdr.h (e.g. IDC_CREATEFILESSPARSE)
• wxCheckBox in the Files page in muuli_wdr.cpp, with a tooltip
• move the pref in Preferences.cpp:1674 from s_MiscList.push_back(...) to NewCfgItem(IDC_CREATEFILESSPARSE, new Cfg_Bool("/eMule/CreateSparseFiles", s_createFilesSparse, true))

No EC work is required: the tag is already serialized (ECSpecialMuleTags.cpp:352), applied (:766), and EC_PREFS_FILES is already in amuleGUI's exchange mask (amule-remote-gui.cpp:1152), so the control round-trips in remote mode for free.

Note the sense: the stored key is sparse (true by default), while the accessor used by EC is the inverted CreateFilesNormal() (Preferences.h:636). Whichever polarity the label uses, it must match the widget binding.

Prior art — the Web UI already ships this

The amuleapi Web UI exposes the same pref as files.create_normal on the Files tab, in the Downloads group, immediately after alloc_full_size (src/webapi/static/js/views/preferences.js:129):

{ key: "endgame", type: "bool" },
{ key: "alloc_full_size", type: "bool" },
{ key: "create_normal", type: "bool" },        // <-- here
{ key: "check_free_space", type: "bool" },

with the labels (src/webapi/static/i18n/en.json:662, es.json:662):

"prefs_field_files_create_normal": "Create new files non-sparse (allocate real disk blocks)"
"prefs_field_files_create_normal": "Crear los archivos nuevos sin huecos (reservar bloques reales en disco)"

The Web UI uses the positive create_normal sense (true = non-sparse) because that is what the API and the EC tag carry.

Acceptance criteria

• The checkbox appears on the Files preferences page in both amule and amuleGUI.
• Toggling it and pressing OK persists /eMule/CreateSparseFiles in amule.conf with the inverted value.
• In amuleGUI remote mode the checkbox reflects and updates the daemon's value.
• The new label and tooltip are wrapped in _() and appear in po/amule.pot.