feat(web): implement subkey menu realignment for even columns (#9768)

[MattGyverLee]

This is a pull request to resolve #9768

This change addresses a usability issue where subkey popup menus with
even-numbered columns (2, 4, 6, 8) create ambiguous default selection.
When there's an even number of columns, the centered position falls
between two options, making it less predictable which character will be selected.

The solution shifts the popup menu alignment based on the base key's
position on the keyboard:

• Keys on the right side: shift popup menu left by half a key width
• Keys on the left side: shift popup menu right by half a key width

This ensures one option always sits unambiguously under the default
touch position, following the pattern used by Google's Gboard.

It is noted that the shift may be larger (up to a limit) if the selected key has an unusual width.

Changes:

• Added numColumns instance variable to track column count
• Modified reposition() to apply shift for even-column layouts
• Added comprehensive unit tests for even/odd column scenarios
• Existing bounds checking prevents menu overflow

Closes #9768 (#9768)

User Testing:

1. Open a keyboard with larger popup key menus like EuroLatin.
2. Longpress the g, which has 6 glyphs in the popup (the code only activates when there is an even number of rows).
3. If this code is working, the popup should have shifted to the side so that one key is unambiguously selected. If not, sliding your finger left and right over the centerline will toggle between 2 keys.

[keymanapp-test-bot]

User Test Results

Test specification and instructions

ERROR: user tests have not yet been defined

[keyman-server]

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