Refactor filter name replacements

Requested changes:

1. Change usage of <filter> to $filter; optionally support the ${filter} syntax as well.
2. Change usage of <source> to $source; optionally support the ${source} syntax as well.

Reasoning

• Modernize replacement tokens for clarity and convention.
• ${var} is common in templating and allows for easier future enhancement.

Acceptance Criteria

• All locations in the code that use <filter> are replaced with $filter and optionally support ${filter}.
• All locations that use <source> are replaced with $source and optionally support ${source}.
• Ensure that documentation and user-facing strings, as well as internal variable replacements, are updated.
• No breaking changes for NDI naming features.
• Unit/integration tests should pass and/or be updated if they verify legacy syntax.

References

• See usages in src/ndi-filter.cpp within ndi_sender_create. Current logic supports text replacement for <source> and <filter> tokens in NDI names.

Example context:

// Old:
// if (ndi_name.contains("<filter>")) {...}

// Target change:
// if (ndi_name.contains("$filter") || ndi_name.contains("${filter}")) {...}

Enhancement type: refactor