feat: Skills.sh marketplace integration + navigation restructure

Skills.sh Marketplace:
- Add marketplace browser with search, detail panel, and install/uninstall
- Search API proxying skills.sh with lock file integration for installed state
- Install/remove APIs using `npx skills` CLI with SSE streaming progress
- SKILL.md content fetching via GitHub Git Trees API with in-memory caching
- MarketplaceSkillCard, MarketplaceSkillDetail, MarketplaceBrowser components
- InstallProgressDialog with real-time SSE log output
- SkillsManager gains "My Skills" / "Marketplace" segmented control
- Type definitions: MarketplaceSkill, SkillLockFile, SkillLockEntry

Navigation restructure:
- Split Extensions page into standalone /skills and /mcp top-level pages
- NavRail: replace single Extensions icon with separate Skills (ZapIcon) and MCP (Plug01Icon) entries
- /extensions page now redirects to /skills or /mcp based on ?tab param
- /plugins redirects updated to point to new routes
- Remove redundant source badges from SkillListItem (section headers suffice)

Other:
- Add WeChat user group QR code to README.md and README_CN.md
- i18n: add 19 marketplace-related keys in en.ts and zh.ts
- Bump version to 0.18.0

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/types/index.ts

@@ -232,6 +232,33 @@ export interface SkillDefinition {
   enabled: boolean;
 }
 
+// --- Marketplace (Skills.sh) ---
+
+export interface MarketplaceSkill {
+  id: string;
+  skillId: string;      // e.g. "git-commit"
+  name: string;
+  installs: number;
+  source: string;       // e.g. "owner/repo"
+  isInstalled?: boolean;
+  installedAt?: string;
+}
+
+export interface SkillLockFile {
+  version: number;
+  skills: Record<string, SkillLockEntry>;
+}
+
+export interface SkillLockEntry {
+  source: string;
+  sourceType: string;
+  sourceUrl: string;
+  skillPath?: string;
+  skillFolderHash: string;
+  installedAt: string;
+  updatedAt: string;
+}
+
 export interface CreateSkillRequest {
   name: string;
   description: string;