[FEATURE] Wallpaper panel: open at the current wallpaper and preview selections before applying

Submission checklist

• I have searched existing issues and confirmed this has not been requested before.
• I have checked existing pull requests for similar changes.

Feature type

UI / visual improvement

Feature summary

Two related changes to the Wallpaper panel:

1. When the panel opens, scroll the grid so the wallpaper currently applied to the selected monitor is visible. The tile is already marked as current, but the grid always starts at the top, so on a large folder that highlight is off-screen.
2. Make a tile click a preview instead of a commit: apply it to the outputs so it can be judged full-screen, but keep it revertible, with Apply/Done and Cancel in the toolbar. Cancel restores the wallpaper that was set when the panel opened.

Motivation / use case

With a few hundred wallpapers across three monitors, I lose my place on every visit: reopening the panel drops me back at the top of the list with no cue to where the current wallpaper sits, so I scroll hunting for it.

And because a click writes the path to settings.toml and applies it right away, there is no way to try three or four candidates and back out. The wallpaper I started from is only recoverable by finding its tile again, which is the same hunt.

Proposed solution

• On open, scroll to the current wallpaper's index and keep the existing "current" highlight. VirtualGridView::scrollToIndex() is already used for keyboard selection.
• Treat tile clicks as a preview applied to the live outputs but not persisted. Apply/Done persists the choice; Cancel restores the wallpaper from panel open. Whether closing the panel without choosing means apply or cancel is a judgement call I'd leave to whatever matches Noctalia's conventions elsewhere.

Alternatives considered

Starring candidates and comparing them among the favourites pinned at the top — still commits every change, and still starts at the top of the grid.

References / related projects

• #4047 — keeping ephemeral wallpaper changes out of the main settings file. A preview state would be ephemeral by definition, so the two fit together.
• #3898 — opening the wallpaper selector with the current monitor selected; same "open in a useful state" theme, different field.
• #2189 (v4, closed) — asked for the palette-mode list to stay open and show which entry is selected while comparing.

Additional context

Noctalia 5.1.0 (Arch/CachyOS package), niri, three 1920x1080 monitors.