feat: extract wallpaper accent without touching the pywal cache

Calls pywal.backends.colorz.get() directly rather than running `wal -i`. That
function returns a list and writes nothing, so the preset-driven cache that
kitty, dunst and neovim read is left alone. `wal -i` has no isolation flag and
would rewrite all of ~/.cache/wal, reintroducing the unreadable-terminal
problem the preset was adopted to fix.

Writes atomically, so a rofi launch concurrent with a wallpaper change cannot
read a half-written theme. A missing or broken image falls back to mauve rather
than leaving no accent at all.

Verified across six wallpapers with colors.json byte-identical afterward.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G6aeE37K4GHBsaM51yLTTq

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