Extend text zoom range to 50–300%

Extend the user text-size range from 67–200% to 50–300%, allowing smaller text in compact windows and larger text when preferred. Add 50%, 250%, and 300% to the existing zoom ladder; the default remains 100% and the existing intermediate steps are retained.

Use the existing persistence, keyboard, and Ctrl+scroll handling, and document the wider range in the README. This proposes wider fixed limits for the same text/interface scaling behavior already used by Omawrite.

Validation

• ./bin/test on this feature branch: 94 passed, 0 failed.
• ./bin/build: successful; git diff --check passes.
• Regression coverage checks both endpoints, persistence into another backend, repeated input at the limits, and actual Ctrl+- / Ctrl+0 keypresses.
• A separate offscreen check confirmed 10px and 60px editor fonts at 50% and 300% with the desktop scale at 100%, saved zoom values, unchanged document contents and selection, and reset to 20px with Ctrl+0.