feat: add Save All support for tabs and close confirmation

[TechSwimmer]

Save All (UX Improvement)

Closes #82

SnapDock now supports Save All for multi-tab workflows.

What was added

Toolbar action

• Added a Save All button next to Save
• Saves all open tabs with unsaved changes
• Skips clean tabs automatically
• Updates tab save-state indicators after save

Close dialog action

The unsaved-changes prompt now includes:

• Cancel
• Save All
• Discard Changes

Save All behavior

• Saves all dirty tabs before closing SnapDock
• No per-tab discard prompts in this flow
• App closes only after successful saves

Core implementation

• Added save_all_tabs() in src/modules/file/tabs.js
• Wired toolbar action in src/modules/ui/editorSync.js
• Added close-flow IPC bridge:

src/preload.js

• workspaceAPI.onSaveAllForCloseRequest(...)
• workspaceAPI.sendSaveAllForCloseResult(...)

src/scripts.js

• Handles close-time save-all request
• Executes save_all_tabs()
• Returns result to main process

main.js

• Updated close orchestration to support Save All decision flow

Notes / Current Limitations

• Untitled dirty tabs may open Save As dialogs (expected behavior)
• Batch save is best-effort (no transactional rollback)
• Manual feature only (no autosave yet)

Manual Testing

• Tested toolbar Save All with multiple dirty tabs
• Verified clean tabs are skipped
• Verified save-state indicators update correctly
• Tested close dialog Save All flow
• Verified app closes after successful save
• Tested in single-file and folder workspace modes