feat: add SQL file open, save, and save-as (#475)

[datlechin]

Summary

• Open .sql files via Cmd+O (NSOpenPanel)
• Save file-backed tabs via Cmd+S (writes back to source file)
• Save As via Cmd+Shift+S (NSSavePanel for new or untitled queries)
• Native macOS window title bar integration: proxy icon, unsaved-changes dot
• File-dirty tracking on query tabs
• Fix: sourceFileURL was lost on app restart (persistence bug)

Closes #475

Changes

• SQLFileService (new): stateless file I/O service — read, write, open panel, save panel
• QueryTab: added savedFileContent + isFileDirty for dirty tracking
• TabPersistenceCoordinator: fixed convertToPersistedTab() dropping sourceFileURL
• MainContentCommandActions: extended saveChanges() decision tree, added saveFileToSourceURL(), saveFileAs(), openSQLFile(); hasUnsavedChanges now includes file dirty state
• TableProApp: "Open File..." (Cmd+O) and "Save As..." (Cmd+Shift+S) menu items
• KeyboardShortcutModels: new .openFile and .saveAs shortcut actions
• MainContentView: toolbar Save button enables for file-dirty tabs; window.representedURL and window.isDocumentEdited for native title bar
• MainEditorContentView: updates isDocumentEdited on query text change

Test plan

• Cmd+O → NSOpenPanel appears, selecting .sql file opens it in new tab
• Edit content in file-backed tab → title bar shows unsaved dot, Save button enables
• Cmd+S on file-backed tab → file written to disk, dot disappears
• Cmd+Shift+S on untitled query → NSSavePanel, file created, tab becomes file-backed
• Cmd+click proxy icon in title bar → reveals file path in Finder
• Close tab with unsaved file changes → confirmation dialog appears
• Quit + relaunch → file-backed tabs restore correctly
• Double-click .sql in Finder → opens in TablePro (existing, still works)
• Open same file twice → deduplication (focuses existing tab)
• Cmd+S with data grid changes → saves grid, not file (grid takes priority)