filemanager: fix mouse click handling, crashes, and header-click edge case

[mp3cko]

Summary

• Rework mouse click handling in the tree view to use pre/post handlers (preMousePress/onMousePress), letting Micro map the click to the correct buffer position instead of manually computing it — fixes incorrect click targeting when softwrap is enabled
• Add nil checks around scanlist[y] and tree_view to prevent panics on stale/out-of-range clicks
• Disable autocomplete in the tree view so Tab navigation works again (previously swallowed by the autocomplete popup)
• Silently ignore clicks on the header (current dir path) and separator lines instead of erroring
• Reduce default tree pane width from 30 to 25
• Document the above in CHANGELOG.md under Unreleased

Test plan

• Manually reviewed diff for correctness and consistency with existing plugin patterns
• Manual smoke test in Micro: open tree, click files/dirs/header/separator/.., verify Tab navigation and no crashes