feat: Add custom system prompt support for LLM text formatting

[KojoBarbie]

Summary

The LLM-based text formatting pipeline currently uses a hardcoded system prompt and application-type-specific rules defined in formatter-prompt.ts. Users have no way to add their own instructions to influence how the LLM formats transcribed text.

Current Behavior

• The system prompt ("You are a professional text formatter...") and application-type-specific rules (email, chat, notes, etc.) are hardcoded in apps/desktop/src/pipeline/providers/formatting/formatter-prompt.ts
• FormatterConfig only exposes enabled, modelId, and fallbackModelId
• The Settings UI only allows toggling formatting on/off and selecting a model

Proposed Change

Allow users to define a custom system prompt that is appended to the existing built-in system prompt when sent to the LLM. The built-in prompt and application-type rules remain unchanged and always apply — the custom prompt provides additional user-specific instructions on top of them.

Example use cases

• “Always use polite language when writing.”
• “Leave technical terms in English.”
• “Use bullet points frequently.”
• Domain-specific instructions (medical, legal, etc.)

Implementation scope

1. Type/Schema: Add an optional customSystemPrompt field to FormatterConfig in types/formatter.ts
2. Database: Update AppSettingsData in db/schema.ts to persist the custom prompt
3. Prompt construction: Update constructFormatterPrompt() in formatter-prompt.ts to append the user-defined prompt after the built-in prompt when present
4. tRPC router: Update setFormatterConfig mutation in trpc/routers/settings.ts to accept the new field
5. Settings UI: Add a textarea to FormattingSettings.tsx for editing the custom system prompt, with a clear/reset option

Behavior

• Custom prompt empty (default): Existing behavior — only the built-in system prompt is sent
• Custom prompt set: Built-in system prompt + custom prompt are concatenated and sent together to the LLM
• The built-in prompt is never replaced or overridden by the custom prompt

Happy to contribute a PR if this is something you'd accept.

[haritabh-z01]

@KojoBarbie We will be adding prompt customisation soon. It's on our immediate roadmap.

[nchopra]

Update: We are starting work on this, planning to release in beta in v1.1.0 soon.