feat: add Cmd+/Cmd- editor font zoom

[datlechin]

Summary

• Added Cmd+= (Zoom In) and Cmd+- (Zoom Out) shortcuts in the View menu
• ThemeEngine.adjustEditorFontSize(by:) adjusts the editor font size in-memory (session-only, not persisted to disk), clamps to 9-24pt, and rebuilds the editor font cache
• Updated keyboard shortcuts docs

Test plan

• Press Cmd+= multiple times — editor font grows, caps at 24pt
• Press Cmd+- multiple times — editor font shrinks, caps at 9pt
• Quit and reopen — font size resets to theme default (session-only zoom)
• Verify data grid font is NOT affected (only editor zoom)