feat: add ${custom_words} placeholder to post-processing prompts

[djurcola]

Summary

• add ${custom_words} placeholder support to post-processing prompt templates
• render custom words consistently for structured-output, Apple Intelligence, and legacy provider paths
• use an explicit (none provided) fallback when the custom-word list is empty
• preserve placeholder-like text inside transcription and custom-word values without recursive expansion
• document ${output} and ${custom_words} in the English prompt UI

Motivation

Custom words already help transcription correction, but post-processing prompts cannot currently reference the configured list. This lets users include the same domain-specific vocabulary in their LLM instructions without duplicating it manually.

Testing

• cargo fmt --check --manifest-path src-tauri/Cargo.toml — passed
• focused pure-Rust renderer harness — 5 passed
• npm run lint — passed
• npm run build — passed
• Prettier check for src/i18n/locales/en/translation.json — passed
• JSON parse validation — passed
• git diff --check — passed

The full Cargo test target could not compile on the local Linux host because pkg-config/GLib development tooling is unavailable; it failed while building glib-sys, before compiling Handy. The focused renderer was therefore compiled and exercised independently with rustc --test.

Compatibility

• existing ${output} behavior is preserved
• templates without either placeholder are unchanged
• existing fuzzy custom-word transcription correction is untouched
• no settings schema, generated bindings, CI, or dependency changes

AI assistance disclosure

Yes. OpenCode using the Codex-backed GPT-5.6 Sol model assisted with adapting the existing fork implementation to current upstream, and Hermes Coder reviewed the diff and ran verification. The final changes were constrained to the two feature-relevant files.

[cjpais]

This is probably a good change, I will try and merge it when I get a chance

[djurcola]

I am running it locally and it makes post processing almost perfect, would be great to get it into the main branch!