fix: fix generic interface crash and resolve package main interfaces

[SangTran-127]

#8 Fix 'modifiable is off' (Vim:E21) crash when confirming generic interface:

• Explicitly pass target bufnr and use vim.fn.appendbufline instead of vim.fn.append.
• Cleanly unmount Nui layout before resuming coroutine and restore window focus.
• Default generic argument to type parameter name instead of empty string.
• Fix 'Failed to add the implementation' for interfaces in package main or the current package by passing the interface name directly without package prefix.
• Expose impl stderr in error notifications for clearer diagnostics.