Remove redundant Save Theme button causing theme creation failures

[Copilot]

Themes failed to save when edited from the default "Dark" theme. The "Save Theme" button in the Appearance tab threw silent errors while a separate working implementation existed in the main Save handler.

Changes

Removed duplicate theme creation path:

• Deleted handleSaveTheme function and "Save Theme" button from AppearanceTab
• Removed onSaveTheme prop, savingTheme, and saveError state
• Consolidated to single save flow through main Settings handler

Updated user guidance:

• Hint now reads: "Theme changes will be saved when you click 'Save' at the bottom"
• Single Save button at dialog bottom handles both settings and theme creation

Implementation

The main handleSave already correctly:

• Detects color/name modifications by comparing with base theme
• Creates custom themes via window.pdv.themes.createCustom()
• Persists theme name changes
• Works for both default and selected themes

The removed handleSaveTheme duplicated this logic with bugs (required pendingThemeColors upfront, causing failures when editing default theme). Keeping the working implementation eliminates the UX confusion and code duplication.

Net: -100 lines

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[matt-pharr]

@copilot very briefly summarize what is added in this pull request.