Summaries through OpenAI and Anthropic, with an editable prompt

[thesiti92]

Closes #788. Bundles diffr 0.1.9 (devdotfast/diffr#60, #62 and #65).

Settings → AI summaries

• Provider: Gemini, OpenAI (also any OpenAI-compatible server) or Anthropic. Switching resets the model to that provider's default.
• Endpoint URL: optional. Leave it empty for the provider's own API.
• Prompt: editable, showing whether it is the default or customized, with "Reset to default". The description says diffr asks for structured output where the provider supports it.
• Test setup: uses the draft provider, endpoint and prompt, so you can try them before saving.
• Keys: switching providers clears a key saved for the previous one, so a key is never sent to another vendor. Credentials are checked against the chosen provider's environment variable.

No duplicated defaults. Provider ids, titles, default models, endpoints, key variables and the keyless rule all come from diffr config schema. A default changed in diffr, or a provider added there, needs no change here.

Server

• Reads the default prompt from diffr config schema.
• Writes only settings that changed. diffr now drops values that equal their defaults, so resetting the prompt removes it from the config file.

Verified

• Unit tests (19) and browser tests (10) pass.
• Against a local diffr build with the same code:
  • switching provider clears the old key and keeps the config file sparse;
  • resetting the prompt removes it from the file;
  • Test setup works through a keyless OpenAI-compatible server;
  • Test setup works live against Anthropic (claude-sonnet-5-5).
• Checked in an isolated dev Desktop; that check caught two UI fixes.
• test:integration:diffr (16) passes against the pinned 0.1.9 release.