Add Ungoogled Chromium template (native theme extension) using the Noctalia palette

[venus-fire]

Template

• App: Ungoogled Chromium
• Template id (directory): ungoogled-chromium
• New template

What it themes

Chromium has no userChrome.css, so this template uses Chromium's native
theming mechanism — a theme extension (manifest.json with M3 colors and
tints). Noctalia renders the manifest into a stable path
($XDG_CACHE_HOME/noctalia/ungoogled-chromium/theme/manifest.json) on every
palette change; the browser loads that folder as an unpacked theme.

Colors themed: window frame, tab strip / toolbar, bookmark bar text, new tab
page background/text/link.

Because a static theme can't follow prefers-color-scheme, the manifest uses
default-mode tokens (the user's current Noctalia mode); switching dark/light
re-renders the manifest and a browser restart applies it.

Testing

Tested on this machine (Noctalia v5.0.0). The theme was loaded via
--load-extension and confirmed registered as the active theme
(extensions.theme.id + pack set in the profile Preferences), then
screenshotted.

• App version tested: ungoogled-chromium 151.0.7922.71
• Applied a dark theme and confirmed the app picked it up (screenshots below)
• Applied a light theme and confirmed the app picked it up (mode was dark during testing)
• Re-applied twice and the app config is still correct (hooks are idempotent)

One-time manual step: load the theme folder unpacked from chrome://extensions
(Developer mode → Load unpacked), which is the standard way to install
extensions in ungoogled-chromium. After that the stable path keeps receiving
updates, so no further manual steps are needed per palette change.

Screenshots

Hooks

• What the hook does: ensures the theme directory exists and prints a
  one-time install notice (idempotency marker: INSTALL.txt); it downloads
  nothing and touches only the template's own cache directory.
• It is idempotent: running it twice does not duplicate lines or corrupt the config.
• It fails loudly (non-zero exit, message on stderr) when the app's config is missing.
• It downloads nothing and executes nothing it did not ship in this directory.
• It touches only this app's own config.

Checklist

• The directory name matches the [catalog.ungoogled-chromium] id in template.toml.
• category is one of: ai, audio, browser, chat, compositor, editor, gaming, launcher, system, terminal, misc.
• Every input_path names a file that exists in this directory.
• No generated output or app-specific personal config is committed.