refactor(dirvish): use cj/executable-find-or-warn in cj/set-wallpaper

Migrate `cj/set-wallpaper' to the new helper.  The old code messaged a bare `"%s not found"' when feh or swww was missing -- the helper produces a better message ("feh not found; wallpaper setter unavailable") that lands in *Warnings* instead of flashing once in the echo area.

Add `(require \='system-lib)' to dirvish-config.el per the Phase 2 exit criterion.

Audit notes for the other `executable-find' call sites in prog-shell, prog-c, prog-go, prog-python, browser-config: all silent `:if' / LSP-availability checks (spec says keep silent), or interactive commands with platform-specific install hints in their existing messages (`shellcheck`, `mypy`, `delve`).  Migrating those would lose the install hints; leaving them.

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