feat: optional tauri auto-updater behind cargo feature

[eleboucher]

Add cargo feature (on by default) so downstream builders can disable the Tauri auto-updater with --no-default-features --features wry.

• tauri-plugin-updater becomes an optional dependency
• plugin registration gated by #[cfg(all(desktop, feature = updater))]
• updater:default permission moved to its own capabilities/updater/updater.json
• build.rs selects the capability glob based on the feature, since tauri-build fails on permissions from missing plugins

Description

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[7w1]

conflicts D: