feat(rpc): Export GUI Policy Options for Headless Deployments

[kwsantiago]

Summary

Add RPC commands to export/import GUI policy settings for headless systems (Start9, Umbrel).
Relates to #130

Changes

• dumpsettings: Export settings to JSON with optional detailed metadata (replaces separate getsettings)
• getsettingsschema: Generate JSON Forms schema
• setsettings: Update individual settings (renamed from setsetting)
• updatesettings: Bulk atomic updates
• subscribesettings: Change notifications

Security & UX improvements:

• Sensitive settings always masked (no include_sensitive parameter)
• Enhanced GUI import preview with danger warnings and countdown timers for critical settings
• Category-based permissions (network, rpc, wallet, mempool, etc.)
• No rate limiting (users control their own nodes)

GUI export/import buttons and ZMQ notifications included.

Test plan

• RPC commands work with bitcoin-cli
• GUI export/import functionality
• Functional and unit tests pass

[luke-jr]

There's a lot to review here, so this isn't complete comments yet.

[kwsantiago]

Thank you for taking a look @luke-jr I greatly appreciate it.

This is still a WIP and there's more I'd like to do here, but your comments are really helpful as I work through this to ensure it meets Knots' standards.

[luke-jr]

One issue is going to be review. This is largely one giant commit, which makes it difficult. Can you break it up into smaller steps?

You're also putting a lot in the RPC code. At the end of this, we should ideally have RPC and GUI using the same code for changing settings. So an internal interface for updatesettings called by both would be appropriate. There's a lot of existing parsing code that you can probably reuse too (eg, boolean string to boolean UniValue conversions...)

[kwsantiago]

@luke-jr for review purposes, would it be better to break this into several PRs, or would you rather have everything in one PR?

[luke-jr]

Multiple PRs could make sense, but there's tradeoffs. As long as it's broken up into logical separate commits, that's probably good enough.