fix(keymap): make modifier layers discoverable

[blackbartblues]

Plugin

• Id: blackbartblues/keymap
• New plugin
• Update to an existing plugin (version bumped in plugin.toml)

What it does

• Correctly identifies a selected key as used on another modifier layer instead of presenting the current combination only as free.
• Lists the Super/Ctrl/Shift/Alt layers that contain shortcuts for the selected key.
• Lets users jump directly to a matching layer from the keyboard details card.
• Documents the layer-navigation behavior and bumps Keymap to 1.3.6.

Closes #203.

External dependencies

No dependencies added.

Testing

• Tested on Niri
• Tested on Hyprland
• Tested on Sway
• Tested on another compositor: automated parser, scale, writer, i18n, manifest, and lint checks
• Noctalia version tested against: v5 current local installation (offline plugin lint)
• Plugin API level: 9

Commands run:

for test_file in tests/*.lua; do lua "$test_file"; done
python3 tests/i18n_test.py
python3 tests/command_library_test.py
python3 .github/workflows/scripts/validate-plugins.py
python3 -m unittest discover -s .github/workflows/scripts -p 'test_*.py'
noctalia plugins lint keymap

All checks passed; the Noctalia linter reported 0 errors and 0 warnings.

Screenshots / Videos

Not attached yet; this remains a draft pending a runtime screenshot of the new layer buttons.

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.
• 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

Plugins run as trusted, unsandboxed Luau in the user's session. Confirm:

• 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.