feat(skill): add named and colored Skill groups

[MicroSharpAnt]

Summary / 概述

• add persistent named and color-coded Skill groups with one-group-per-skill membership and an Ungrouped section
• add collapsible group sections, rename/color/member management, per-app group toggles, and group-level updates
• keep search results visible without overwriting locally saved collapse state
• persist groups through a schema v19 migration, backup/sync/import/export flows, and add frontend/backend coverage
• update all four locale files (en, zh, zh-TW, ja)

Related Issue / 关联 Issue

Fixes #7007

Related to #1890. Complements the source filtering/search/generic batch work in #3869 with persistent user-defined grouping.

Screenshots / 截图

Before / 修改前	After / 修改后
Flat installed-Skills list

Verification / 验证

• pnpm typecheck

• pnpm format:check

• pnpm exec vitest run tests/components/UnifiedSkillsPanel.test.tsx (36 passed)

• cargo fmt --check --manifest-path src-tauri/Cargo.toml

• cargo test --all-targets --manifest-path src-tauri/Cargo.toml (2765 library tests passed; integration suites passed)

• production Tauri application built, installed, and exercised against a v18 database; automatic migration to v19 completed and PRAGMA quick_check returned ok

Upstream baseline checks

• pnpm test:unit: 1019 passed; 2 unchanged upstream assertions fail because grok-4.5 now declares xhigh while tests/config/codexReasoningLevelPresets.test.ts still expects only low/medium/high
• cargo clippy --manifest-path src-tauri/Cargo.toml -- -D warnings: this change adds no warnings, but the command is blocked by four existing Rust 1.98 warnings in unchanged files (src/gemini_mcp.rs, src/proxy/forwarder.rs, and src/tray.rs)

Checklist / 检查清单

• pnpm typecheck passes / 通过 TypeScript 类型检查
• pnpm format:check passes / 通过代码格式检查
• cargo clippy passes (if Rust code changed) / 通过 Clippy 检查（如修改了 Rust 代码） — blocked by the unchanged upstream warnings documented above
• Updated i18n files if user-facing text changed / 如修改了用户可见文本，已更新国际化文件