Improve startup scripts, server debug option, and connection help text

[TheZupZup]

Motivation

• Make local startup scripts more robust and user-friendly by adding strict error handling and explicit path checks.
• Provide clearer guidance in the app's connection screen about how to start the backend (including a provided helper script).
• Allow enabling the WebDAV debug middleware when the optional package is available to ease troubleshooting.

Description

• Update ConnectScreen help text and code snippets to recommend bash nexanote.sh and use a generic cd <your-folder>/NexaNote path, and adjust the unreachable-server message.
• Harden launch_app.sh by adding set -euo pipefail, resolving the script directory, validating the built Flutter binary, and emitting actionable instructions if the binary is missing.
• Improve nexanote.sh by adding set -euo pipefail, computing SCRIPT_DIR, detecting and sourcing the virtualenv if present, starting the backend in the background with logging, adding a wait loop and trap-based cleanup to kill the backend on exit, and switching to flutter run for development.
• Make nexanote.sync.server.build_app conditionally insert the wsgidav.debug_filter.WsgiDavDebugFilter into the middleware stack when the wsgidav.debug_filter module is importable using importlib.util.find_spec.

Testing

• No automated tests were added or executed as part of this change.

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Codex Task