add support for 3rd party API's besides OpenRouter

[SellPL]

this pull request adds support for other API's. Worked for me on android with 3rd party provider with claude opus 5 model. more from Claude Fable 5 about this since it was vibecoded:
PR notes:
▎ Add Anthropic (Claude) as a selectable AI provider
▎
▎ Anthropic speaks its own Messages API rather than OpenAI chat completions, so this adds a small protocol discriminator instead of pretending it's another base URL:
▎ - New AiApiProtocol enum; provider registry entry for Anthropic (/v1/messages, key hint sk-ant-..., suggested model claude-opus-5).
▎ - AIService branches per protocol: x-api-key + anthropic-version: 2023-06-01 headers, system prompt moved to top-level system, required max_tokens (2048), OpenAI-style data-URL images converted to base64
▎ image content blocks, reply text taken from the first content[].text block.
▎ - No response_format on Anthropic — relies on the existing strict-JSON prompt and the lenient parseModelJson recovery, unchanged.
▎ - anthropic-dangerous-direct-browser-access: true header so the web build works (BYO-key in browser is already this app's model on every provider).
▎ - Settings UI picks the provider up automatically from kAiProviders; public AIService API unchanged, so no caller edits. README/settings copy updated; unit test added for endpoint/protocol resolution.
▎
▎ Tested: flutter analyze clean, full test suite passes, web + Android release builds compile. Not tested: live calls against the Anthropic API (no key available) — request/response shapes follow current
▎ Messages API docs and want one manual verification.

[riki137]

Thanks for the contribution. The protocol-discriminator approach makes sense, and the Anthropic request shape broadly matches the Messages API. However, this branch is not merge-ready; there are clear signs the generated changes were not properly reviewed.

• The history commits a 66.9 MB APK, the complete web build, and node_modules, then deletes them later. Please rewrite/squash the branch before a merge/rebase so these blobs do not enter the repository history.
• The final diff still contains unrelated package.json and package-lock.json files with an unused git npm dependency. These should be removed. Please also revert or explain the unrelated android/gradle.properties and pubspec.lock changes.
• .gitignore:56-57 contains "node_modules/" and "dist/". Git treats those quotes literally, so the directories are not ignored. Use node_modules/ and dist/ without quotes and clean up the trailing whitespace/line endings.
• Switching from another free-text provider to Anthropic retains the previous model and fallback values: updateProvider() only changes the provider ID. For example, switching from OpenAI with gpt-5.5 and saving will send gpt-5.5 to the Anthropic endpoint. Provider changes need to reset or populate the model fields, with a regression test.
• The added test only checks endpoint/protocol resolution. None of the important new behavior is exercised: Anthropic headers, top-level system, image conversion, omission of response_format, or response parsing. Please make HTTP injectable/mockable and test the actual outbound request and representative responses.
• AIService ignores Anthropic’s stop_reason and attempts to parse every HTTP 200 text block as food JSON. max_tokens, refusal, and context-limit responses need explicit handling. This matters particularly because Claude Opus 5 enables thinking by default and thinking counts against the hardcoded 2,048-token ceiling. Anthropic recommends always checking stop_reason: stop-reason documentation, thinking documentation.

I ran the complete Flutter test suite against commit d59b062; it passes. The core idea is usable, but the branch hygiene and missing protocol-level tests need fixing before merge.