fix(webui): fix flex columns ignoring resize, fixes #1323

[mohamadyasser118]

Fixes #1323

I made changes:

• columnSizing initializes as {}, only user-set widths are stored
• columnsWithSizing only overrides a column when the user has resized it, so when applying user width, strip flex and minWidth so the pixel value is respected in both directions

fix.mp4

AI & Automation Policy

• I am the human author and take full personal responsibility for every change in this PR.
• No AI or automated generative tool was used in any part of this PR OR I have disclosed all tool(s) below.
  Drafted with the assistance Claude Sonnet 4.6

Checklist

• Code compiles and existing tests pass locally.
• New or updated tests are included where applicable.
• Documentation is updated where applicable.