Show banner and log node module load failures when registry is empty

[criskb]

Motivation

• Surface startup problems when no nodes are registered by detecting an empty NODE_DEFS after node registration attempts.
• Make debugging easier by collecting module-specific failures and printing the failing module names and errors to the console.
• Provide a visible UI hint so users know to open DevTools when the node registry failed to load.

Description

• Collects detailed failure records (module + error) from loadNodesFromList() and the entries manifest loader and accumulates them via a failures array and recordFailure helper in loadNodes().
• If Object.keys(NODE_DEFS).length === 0 after all loading attempts, injects a small banner into the top bar (or falls back to the left panel head) with the message: "Node registry empty — open DevTools Console." and styles it for visibility.
• Emits console.error for each recorded failure with the module name and associated error when the registry is empty.
• Keeps existing toast/status behavior and updates nodeRegistry.loaded based on the final node count.

Testing

• No automated tests were run for this change.

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Codex Task