Fix/issue 865 create action disabled

[harminius]

Summary

Fixes #865 - at the QGIS welcome screen with no project open, the "Create Mergin Maps Project" toolbar action stays disabled. Opening any project (then optionally closing it) is required to get it to wake up.

Root cause

enable_toolbar_actions (Mergin/plugin.py:291-303) already has the correct special case for the Create button — it should be enabled whenever self.mc and self.manager are both set,
regardless of whether a project is open. The bug is that create_manager() never calls enable_toolbar_actions() after populating self.mc/self.manager at startup. Other state-change paths
(on_config_changed, on_qgis_project_changed) do, but the initial-load path does not — so on a clean QGIS launch the Create button stays at its initial enabled=False state until on_qgis_project_changed fires (i.e. a project is opened).

Fix

One line - call self.enable_toolbar_actions() at the end of create_manager(). The existing mc/manager check then enables the Create button at startup with no project required. The call is idempotent, and the same function is already invoked from the other state-change paths, so there is no behavioural risk elsewhere (the only side effect is a slightly redundant second call from on_config_changed, which is harmless).