chore(tauri): make the devtools plugin opt-in behind a cargo feature

[eleboucher]

Description

tauri-plugin-devtools retains every metadata record for the session (devtools-core's Aggregator::all_metadata is never trimmed, by design, so late-connecting clients get the full history). With tauri-plugin-log feeding it, a heapprofd capture showed 310 MB retained in 45 seconds; on Android the app reached ~1 GB within a minute and was OOM-killed repeatedly.

It is now behind a devtools cargo feature, off by default. Build with --features devtools to get the inspector back. tauri-plugin-log still initialises either way, so logcat output is unchanged.

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).