First-run guided setup, explorer open buttons, and notes streaming fix

[DevMando]

Summary

Release-prep work: a guided first-run experience for brand-new users, a smoother file explorer, and a fix for the app freezing during notes-assistant replies.

What's new (plain language)

1. First launch now sets itself up. Someone who downloads the release zip and runs it for the first time gets a step-by-step walkthrough right in the chat: the app checks for Ollama, offers to install it (one click, via winget) if it's missing, starts it, and helps pick a first AI model from a short list with plain-language size guidance ("~1 GB, fast on any laptop" up to "best local quality, 8 GB+ VRAM"), or the recommended cloud option. Before this, a new user landed on a settings page with an empty URL box and no guidance. The wizard remains available afterward via /setup or a new "Run guided setup" button in Settings.

2. File explorer rows have an open button. Folders open in Windows File Explorer, files in their default app. (Double-clicking a folder couldn't do this reliably — it conflicts with expanding/collapsing the folder.)

3. Fixed: the app froze while the notes assistant was replying. Fast models could produce text quicker than the screen repainted, locking the whole app until the reply finished. Replies now paint on a fixed 10-per-second schedule regardless of model speed, and closing a note properly stops its in-flight request.

4. Docs refreshed for release. README now opens with an end-user install section (download zip, run, no .NET required); CHANGELOG covers all of the above.

Why it matters

This is the last UX gap before v1: the release zip is fully self-contained, so first-run setup was the only place a newcomer could get stuck. With the wizard, the path from download to first AI reply requires zero prior knowledge of Ollama, models, or endpoints.

Scope / risk

• First-run wizard reuses the existing, tested /setup flow and the engine's OllamaSetupHelper — the new code is mostly wiring and menu options.
• Notes fix is confined to the notes reply pump; the main chat streams unchanged.
• No engine (submodule) changes, no config format changes.

Verification

• Built clean (0 warnings/0 errors) with dotnet build on every commit.
• Notes streaming verified live against qwen3.5:0.8b (worst case: fast thinking model) and minicpm-v4.6.
• Suggested pre-merge check: delete/rename %USERPROFILE%.mandocode and launch to see the first-run wizard; ideally run the release zip once in Windows Sandbox for the true fresh-machine test.