fix(wallpaper): support matugen 4.x and TTY-less invocation

[wendeus0]

Summary

Fixes theme regeneration on systems with matugen >= 4.x (current Arch Extra version). Two related bugs prevent the appearance toggle (Light/Dark, color schemes, wallpaper) from updating any color file — the user_settings.json is saved correctly but every downstream stylesheet stays stale, and on the next ignis reload the shell may crash entirely.

Closes #37

Root cause

1. Missing --prefer flag

Matugen 4.x requires --prefer <strategy> when an image has multiple extractable source colors and no TTY is attached. exec_sh_async never has a TTY, so every matugen image invocation aborts with:

Error: Multiple source colors found, no preference was inputted, and a terminal was not detected.
Use --prefer=PREFERENCE to find suitable colors without needing user input.

The asyncio.create_task swallows this silently, so from the user's perspective the toggle "does nothing".

2. --json hex output shape changed

Matugen 4.x emits

{"colors": {"background": {"light": {"color": "#fcfaec"}, "dark": {"color": "#11150c"}}}}

instead of the previous flat "#fcfaec" strings. generatePreviews() writes color_value straight into preview-colors.scss, producing invalid SCSS like

$palette-content-background: {'color': '#fcfaec'};

Sass compilation fails, the stylesheet pipeline dies, and on the next reload the Ignis shell renders without any CSS (Material Symbols become literal text labels, cards lose their background).

Changes

Where	What
8× matugen image invocations	Append --prefer saturation (matches visual intent of the selected scheme without UI input)
2× loops in generatePreviews()	Unbox color_value when it is a dict, reading color / hex / first-value-fallback

Backwards compatible: the dict unbox only triggers when isinstance(color_value, dict) is true, so matugen-bin 3.x and matugen-git (which already returned strings) keep working.

Test plan

• Reproduce on Arch with matugen-bin 4.1.0: confirmed toggle was a no-op and ignis reload crashed Ignis with Expected expression from preview-colors.scss
• Apply patch, run matugen image -t scheme-expressive '<wallpaper>' -m light --prefer saturation manually — ~/.config/ignis/colors.scss, ~/.config/gtk-3.0/colors.css, ~/.config/gtk-4.0/colors.css, ~/.config/qt5ct/colors/matugen.conf, ~/.config/qt6ct/colors/matugen.conf all regenerate cleanly
• preview-colors.scss now contains valid SCSS ($palette-content-background: #fcfaec;)
• ignis reload succeeds; Light/Dark toggle applies in real time; color scheme bolinhos update palette
• Confirm on matugen-bin 3.x / matugen-git (relying on isinstance guard for compat — would appreciate a second pair of eyes here)

Notes for reviewer

• --prefer saturation is a heuristic choice. If you'd rather expose this as a setting (user_settings.appearance.wallcolors.prefer), happy to push a follow-up.
• The dict unbox path only matters for the preview palette generation. The main setDarkMode / setColors flow doesn't read JSON output, only triggers matugen and lets the templates do their thing — so the existing template files do not need changes.

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