daily-wallpaper: add bar widget, panel, and desktop widget for the caption

[jstehn]

Plugin

• Id: nzlov/daily-wallpaper
• New plugin
• Update to an existing plugin (version bumped in plugin.toml)

What it does

Bing and NASA both ship a caption alongside their photo of the day, but the
service only ever used it to build the wallpaper's cache filename — the text
itself was discarded. This adds three pure-view surfaces that show it: a bar
widget (glyph + optional short caption; click opens the panel), a panel with
the full caption, credit, and a "learn more" link, and a matching desktop
widget card. The background service is unchanged in what it fetches; it now
additionally captures and publishes the caption at the point each source
resolves.

• Bing's copyright field (e.g. "Caption text (© Photographer/Source)") is
  split into a title and a credit.
• NASA's gallery-item caption has no separate credit line, so credit stays
  empty; its own article link is used as "learn more" instead of Bing's quiz
  link.
• Metadata is (re-)fetched even when today's image is already cached (e.g.
  after a process restart before this session ever resolved a source), so
  the caption doesn't stay blank until the next day's fetch.

Also fixes a real bug found while testing the NASA path, unrelated to the
caption feature itself: the pre-existing primary-image regex scanned the
whole page with a lazy .- span, and intermittently exceeded Noctalia's
per-callback CPU budget against the real nasa.gov page (confirmed live —
same line, unmodified from main, before I touched it). All three NASA
extractions (image, caption, link) are now bounded to a small substring
around the first gallery item instead.

External dependencies

Adds xdg-open (from xdg-utils) for the panel's "Learn more" button, opening
the caption's source link. Already declared in dependencies in plugin.toml.

Testing

• Tested on Hyprland
• Tested on Niri
• Tested on Sway
• Tested on another compositor:
• Noctalia version tested against: 5.0.0
• Plugin API level: 3

Tested live via a path plugin source pointing at this branch, on both
source = "bing" (the default) and source = "nasa": noctalia plugins lint and the repo's own validate-plugins.py both pass; the existing
wallpaper-download/apply behavior is unaffected for either source
(Daily Wallpaper applied: ... logs and the wallpaper is actually set,
confirmed by screenshot for both); the today state publishes the correct
title/credit for both sources on a fresh resolve and on a forced refresh
via noctalia msg plugin nzlov/daily-wallpaper:service all refresh; the
panel opens/closes with no runtime errors for either source; the NASA
CPU-budget fix was confirmed by reproducing the original error live, then
confirming it's gone after the fix, across several repeated refreshes.

Screenshots / Videos

Not yet included — opening as Draft for that reason. I have real
screenshots from live testing (bar widget + desktop widget on Bing, panel
open on both Bing and NASA), but I'd rather have a person look at what's in
frame before anything from a live desktop goes into a public PR. Will add
before this comes out of Draft.

Checklist

• The directory name matches the part of id after the / in plugin.toml exactly.
• It ships plugin.toml, README.md, thumbnail.webp, and translations/en.json.
• README.md follows the README template, documents every entry id and dependency, and includes exact panel IPC commands and launcher prefixes where applicable.
• I created thumbnail.webp with the thumbnail generator. (Unchanged from the existing plugin — happy to update if maintainers want the new surfaces reflected.)
• version follows semver and is bumped in this PR; plugin_api is the oldest API level this plugin requires.
• Every non-English translation in this PR uses a locale supported by Noctalia core, and I can read, write, and understand that language well enough to review and maintain it (no unreviewed machine/LLM translations).
• I did not edit catalog.toml; CI generates it.
• This PR touches exactly one plugin directory.

Code review attestation

• The code is readable and not obfuscated, minified, or generated.
• It does not download and execute remote code.
• Every network call, filesystem write, and spawned process is something the description above accounts for.
• I have the right to publish this code under the license declared in plugin.toml.

[github-actions]

CC @nzlov