feat(autobidsify): add public private mode page, local AI support, and Ollama security

[elainefan331]

PR Description:

Summary

• Add public /autobidsify page for non-logged-in users (private mode) — existing save mode in dashboard unchanged
• Overhaul AI Assistant panel UI with clearer labels and live progress output
• Add local AI support (Ollama / LM Studio / Jan) for private mode users
• Secure Ollama backend with auth guard and rate-limited public endpoint

Changes

Public AutoBIDSify Page

• New public page at /autobidsify (renamed from /bids-converter)
• Welcome dialog: choose Private Mode (local only) or Save to Account (redirects to dashboard)
• Private mode: slate grey background, nothing uploaded, data lost on close
• Navbar: AutoBIDSify dropdown with GitHub link and web app link

AI Assistant Panel

• Renamed buttons: "Generate BIDS Metadata Files", "Generate Conversion Package", "Preview Conversion Package in
  File Tree"
• Output panel shows live status messages during generation; clears on step 2 re-run
• Output folder name includes timestamp to distinguish multiple runs
• Warning shown in private mode when using external AI provider

Local AI Support (Private Mode)

• New "Local AI (Ollama / LM Studio / Jan)" provider — direct browser-to-localhost calls
• Configurable URL field with port hints for each tool
• Custom model name input to override dropdown selection
• Ollama (NeuroJSON server) available in save mode only; server provides qwen3.6:27b as the only available model

Ollama Security

• Add auth guard