Feature request: preferred output formats with “Show all” option

Problem

ConvertX exposes every output format supported by every matching converter. This is comprehensive, but the initial list can be overwhelming for users who normally need a small set of common formats.

This is related to C4illin/ConvertX#173 and C4illin/ConvertX#238, but proposes a softer approach: simplify the default UI without permanently blocking access to the complete format catalog.

Proposal

Add an optional environment variable containing a comma-separated list of preferred output formats, for example:

PREFERRED_OUTPUT_FORMATS=jpg,jpeg,png,webp,pdf,mp4,mp3,docx,xlsx

When configured:

1. The output selector initially displays only preferred formats that are valid for the selected input file.
2. A clear Show all formats action reveals the complete existing converter list.
3. Search initially operates on the preferred list. After selecting Show all formats, it searches the complete list.
4. The preference affects presentation only. It is not a security control and does not disable converters or reject manually selected conversions.
5. Unset or empty configuration preserves the current behavior exactly.

Suggested behavior

• Compare extensions case-insensitively.
• Trim whitespace and ignore empty entries.
• Ignore unknown formats while logging a warning at startup.
• Preserve converter grouping when multiple converters support a preferred format.
• Keep the Show all formats choice for the current browser session or current conversion, whichever fits the existing frontend architecture best.
• Continue intersecting the preferred list with formats that are actually supported for the uploaded input.

Example

With:

PREFERRED_OUTPUT_FORMATS=jpg,png,pdf,mp4

Uploading a HEIC image could initially show jpg and png. Uploading a MOV video could initially show mp4. Users could still select Show all formats to access advanced or unusual targets.

Acceptance criteria

• Add a documented optional environment variable for preferred output formats.
• Unset configuration keeps the current full-list behavior.
• Configured preferred formats are shown first by default and filtered against valid conversions for the input.
• Users can reveal and search the full format list without restarting or changing server configuration.
• Invalid configured values do not break startup or conversion.
• Add tests for parsing, default filtering, input-format intersection, and the Show all formats behavior.