fix: show warning instead of validation error on WIP settings tabs

[lane711]

Summary

• Fixes the "Save All Changes" button showing confusing "Site name and description are required" error on non-General settings tabs
• Now shows a warning notification explaining that saving is not yet available for WIP tabs (Appearance, Security, Notifications, Storage)
• General tab continues to work as expected

Root Cause

The saveAllSettings() function was always routing to /admin/settings/general regardless of the active tab. When on non-General tabs, the form didn't contain siteName or siteDescription fields, causing backend validation to fail.

Changes

Added a check in saveAllSettings() that:

1. Maintains a list of tabs with implemented save functionality
2. Shows a helpful warning notification for WIP tabs instead of attempting to save
3. Only proceeds with save when on a tab with working save endpoint

Test plan

• Navigate to Admin > Settings > General tab, make changes, click "Save All Changes" - should save successfully
• Navigate to Admin > Settings > Appearance tab, click "Save All Changes" - should show warning notification
• Repeat for Security, Notifications, and Storage tabs - all should show warning
• Migrations and Database Tools tabs have their own action buttons, so "Save All Changes" should show warning there too

Fixes #345

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