ux: improve Skills and Slash Commands settings UI with multi-mode support

[brunobergher]

Motivation

The Skills and Slash Commands settings panels had inconsistent UIs and skills could only be associated with a single mode. This PR improves the UX consistency and adds flexibility for skill mode targeting. Closes EXT-725

Key Changes

Multi-mode Support for Skills

• Skills now support modeSlugs array in frontmatter (replaces single mode field)
• Skills can target multiple modes or be available in "Any mode"
• Added gear icon button to configure mode associations via a modal with checkboxes
• SkillsManager.updateSkillModes() method to update mode associations without recreating skills

Unified UI for Skills and Slash Commands

• Redesigned SlashCommandsSettings to match SkillsSettings visual pattern
• New CreateSlashCommandDialog component with consistent dialog styling
• Consistent action buttons (edit/delete) and grouping by source (project vs global)

UI Components

• Exported Checkbox, Input, and Textarea from @/components/ui for use across settings panels

i18n

• Added translation strings for all new UI elements across 18 locales

Technical Notes

• Backward compatible: mode field is deprecated but still supported
• modeSlugs takes priority over mode in frontmatter parsing
• Skills are now stored in generic skills/ directory (mode info in frontmatter)

[roomote]

Rooviewer   See task on Roo Cloud

All previously flagged issues have been resolved. No new issues found in this review.

• Fix mismatch between frontend and backend for skill deletion: frontend sends skillModeSlugs but handleDeleteSkill reads skillMode

Previous reviews

• 5792d8f: Review #1
• 9c698f9: Review #2
• 0e1d527: Review #3

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}

[roomote]

Rooviewer   See task on Roo Cloud

All previously flagged issues have been resolved. No new issues found in this review.

• Fix mismatch between frontend and backend for skill deletion: frontend sends skillModeSlugs but handleDeleteSkill reads skillMode

Previous reviews

• 5792d8f: Review #1
• 9c698f9: Review #2
• 0e1d527: Review #3
• 0399189: Review #4

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}