Fix dark/light icon variant for External links in the Settings menu

[cookie-monster1649]

Problem

External links configured with position settings used the wrong icon
variant in Nextcloud's account and Settings menus.

For a paired icon such as audiobookshelf.svg / audiobookshelf-dark.svg,
the base filename audiobookshelf.svg was always registered. Nextcloud's
Settings menu then applies its own theme-dependent filter:

filter: var(--background-invert-if-dark);

In the dark theme this inverts the source icon. Because the source was
already the light-coloured icon, it was inverted to black and became hard
or impossible to see against the dark menu background.

Root cause

External registers the same navigation entry in two places:

• Application::registerSites()
• BeforeTemplateRenderedListener::generateNavigationLinks()

Both supplied the configured base filename for Settings entries, so a fix
in only one file had no visible effect.

Change

For entries of type SitesManager::TYPE_SETTING, the registration code now:

1. Reads the configured icon filename (or external.svg).
2. Derives the paired dark filename by inserting -dark before the extension.
3. Uses it only if that file exists in the app data icon directory.
4. Falls back to the configured icon otherwise.
5. Skips derivation if the admin already chose a -dark. icon (avoids foo-dark-dark.svg).

Header links, guest/login links, public footer links and quota icons are
unchanged. The same block is applied in both registration paths.

Why this works

-dark is the dark-coloured source asset. In the Settings menu the light
theme shows it unchanged, and the dark theme applies core's existing
inversion to produce a light icon — matching core's rendering rather than
adding theme detection inside External.

Scope / non-goals

Does not change header, footer, quota, mobile/desktop client icon selection,
stored site config, or icon upload. Full-colour (non-monochrome) icons can
still be affected by core's inversion; this follows the app's existing paired
monochrome-icon convention and does not claim to solve arbitrary coloured icons.

Follow-ups (not in this PR, to keep it targeted)

• Extract the selection into a shared method/service so the two paths can't diverge.
• Read the icon directory listing at most once per request.
• Unit tests for the pairing/fallback cases.

Standards checked

• DCO sign-off added to the commit
• Coding standard (php-cs-fixer, Nextcloud ruleset) — ran locally, no violations
• PHP lint (php -l) passes on both changed files
• REUSE / license headers — no new files added, existing headers untouched
• info.xml lint — file not touched
• Psalm — not run locally (separate toolchain + baseline); change is trivially typed, will be verified by CI
• PHPUnit — no test changes; no existing unit tests exercise this path

Testing

• Light theme shows the dark-coloured icon correctly.
• Dark theme shows a light icon after core's CSS inversion.
• Account menu receives /apps/external/icons/audiobookshelf-dark.svg instead of audiobookshelf.svg.
• Header / non-Settings entries unchanged; falls back to base icon when no -dark pair exists.
• php -l and php-cs-fixer pass on both changed files.

This change was AI-assisted and validated on Nextcloud 34.0.1.

Fixes #430