Add capability-aware target and converter selection

[DenWin]

Tracked by #9.

Goal

Show only reasonable conversion targets and converter implementations for the selected source files.

Scope

• Build on the existing getPossibleTargets(fileType) capability model.
• Normalize source extensions before resolving capabilities.
• Hide source-to-source conversions unless explicitly useful.
• Select the target format first.
• After a target is selected, show only converters implementing that source-to-target path.
• Provide a recommended/default converter when alternatives exist.
• Put alternative implementations behind progressive disclosure.
• For mixed-source batches, show the intersection of compatible targets or clearly identify which files a target applies to.
• Explain unavailable options where that helps users correct the selection.
• Replace the current large all-converter popup with a concise, searchable control.

Acceptance criteria

• Every offered target is supported for the relevant source file.
• Every offered converter supports the selected source and target pair.
• Mixed-source behavior is deterministic and documented.
• A sensible default converter is selected where possible.
• Capability filtering is covered by automated tests.
• The selection control is keyboard accessible and responsive.

[DenWin]

Testing requirement

Implementation of this issue must follow the red-green-refactor workflow and provide the applicable unit, integration, browser, and real-world evidence defined in #16. Completion requires behavior-level tests and assessment results, not only working production code.