fix: restore file tree width after terminal resize

[KEY60228]

Problem

The file tree sometimes ends up much narrower than file_tree_width and never recovers.

Root cause: the tree window uses winfixwidth, which excludes it from width redistribution when the terminal grows. So whenever the terminal (tmux pane zoom/unzoom, window resize, font size change) momentarily becomes too narrow to fit the tree, Neovim force-shrinks it — and after the terminal grows back, the tree stays at the shrunken width forever. Each shrink ratchets it further down.

Reproduced with an embedded nvim + attached UI:

initial 200 cols   [tree=30* old=84 new=84]
shrink to 25       [tree=21* old=1  new=1 ]
grow back to 200   [tree=21* old=1  new=167]   <- tree stuck, old pane crushed

Fix

• ui/file_tree.lua: re-assert the configured file_tree_width on VimResized (augroup cleaned up in close()).
• ui/init.lua: rebalance the old/new diff panes on VimResized — the same shrink/grow cycle left one pane crushed at winminwidth (width 1) while the other took all the space.
• file_tree.close(): skip nvim_win_close when the tree is the last window (previously raised E444, e.g. pressing q in the tree after :only); deleting the scratch buffer leaves the window on an empty buffer instead.

Verification

End-to-end test driving a real :ReviewThemStart session in an embedded nvim with an attached UI, resizing via nvim_ui_try_resize:

initial 200        [tree=30* old=84 new=84]
shrink to 25       [tree=21* old=1  new=1 ]
grow to 200        [tree=30* old=84 new=84]   <- recovers now
shrink to 20       [tree=16* old=1  new=1 ]
grow to 180        [tree=30* old=74 new=74]   <- recovers repeatedly
tree closed, resize cycle: no stale-autocmd errors
close-as-last-window: ok (no E444)

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