feat: floating save bar in admin panel (Discord-style)

[arediss]

Summary

Replaces the static top save button in admin tabs with a Discord-style floating save bar that slides up from the bottom when unsaved changes are detected.

How it works

• Hidden by default — only appears when values differ from their initial loaded state
• Fixed bottom — always accessible, no scrolling needed
• "Careful — you have unsaved changes!" message with Save and Reset buttons
• Reset restores all fields to their last saved values
• Success state — green styling with checkmark, auto-hides after 3s
• Slide animation — smooth 300ms transition

Reusable component

FloatingSaveBar accepts show, saving, saved, onSave, onReset props — ready to plug into any admin tab.

Applied to

• General tab (change detection on all 9 settings fields + banner)
• AdminTabLayout gets pb-20 padding to prevent bar overlap at bottom of scroll

Other tabs (Notifications, Paths, Roles, etc.) can adopt it incrementally.

Closes #65

Test plan

• Frontend compiles
• Bar appears when changing any setting
• Bar disappears after save with success animation
• Reset restores all values
• Bar doesn't overlap last settings item
• i18n EN + FR