Allow zero to disable minimum window size limits

[culpen90]

Summary

• allow an explicit zero minimum-window fraction to disable Rectangle's screen-relative limit
• use a reusable DoubleDefault for minimum-window fractions while importing legacy float-backed configs
• preserve the existing 25% default when no preference is stored
• allow windows to reach an exact configured limit while retaining a one-point safety floor
• document the zero-value behavior

Root cause

The minimum-size calculation treated zero as invalid and substituted the 25% default. On the reported 2560-point display, the next 30-point resize step from 665 to 635 points was rejected against a 640-point floor before any accessibility resize was attempted.

User impact

Setting minimumWindowWidth or minimumWindowHeight to 0 now lets the application enforce its native minimum size. Existing installations without an explicit setting continue to use Rectangle's 25% default.

Testing

• ChangeSizeCalculationTests: 7 passed
• all tests outside the two existing cooperative-resize test classes: 128 passed
• full scheme: 171 passed; the 10 failing cooperative-resize test cases reproduce unchanged on upstream/main
• git diff --check

Fixes #388