Add chat font size controls

[keilogic]

Summary

• add increase/decrease chat font size actions to the chat toolbar
• persist the chat font size offset in general settings
• refresh visible chat message text panes when the toolbar controls are used

Fixes #594

Validation

• JAVA_HOME=../../tools/jdk-21 ./gradlew.bat --no-daemon compileJava compileKotlin -x test
• git diff --check HEAD~1..HEAD

[keilogic]

Status note: the only failing check currently shown is the Vercel authorization status, which points to a Vercel GitHub authorization URL rather than a code/test failure. The branch is mergeable, and the local validation from the PR body remains the relevant code check: JAVA_HOME=../../tools/jdk-21 ./gradlew.bat --no-daemon compileJava compileKotlin -x test plus git diff --check HEAD~1..HEAD.