refactor(accent): resolve the wallpaper path, finish the atomic-write helper

colors.json recorded whatever path it was handed, so it read either the real
wallpaper or ~/.cache/wal/wpaper, the symlink to it, depending on the caller.
wallp passes the resolved file; a manual run usually passes the symlink. Same
wallpaper either way, so record one spelling.

write_atomic arrived with the quickshell palette but only write_border used
it. The other four writers kept their own copy of the same write-then-rename,
which is the duplication the helper was added to remove, so they use it now.
No behaviour change: the temp file was already created by mkstemp in every
case, so the resulting modes are the same.

Verified by running the script before and after: udt-palette.qml,
udt-accent.rasi and udt-border.lua are byte-identical, and colors.json differs
only in the wallpaper field this commit intends to change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015gbjA2bmswN8jyPDKzrvqe

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