feat: add backlinks display and wiki-link rendering (closes #62, #64)

Implements two key knowledge module features:

**#62 - Backlinks Display:**
- Added BacklinksList component to show entries that link to current entry
- Fetches backlinks from /api/knowledge/entries/:slug/backlinks
- Displays entry title, summary, and link context
- Clickable links to navigate to linking entries
- Loading, error, and empty states

**#64 - Wiki-Link Rendering:**
- Added WikiLinkRenderer component to parse and render wiki-links
- Supports [[slug]] and [[slug|display text]] syntax
- Converts wiki-links to clickable navigation links
- Distinct styling (blue color, dotted underline)
- XSS protection via HTML escaping
- Memoized HTML processing for performance

**Components:**
- BacklinksList.tsx - Backlinks display with empty/loading/error states
- WikiLinkRenderer.tsx - Wiki-link parser and renderer
- Updated EntryViewer.tsx to use WikiLinkRenderer
- Integrated BacklinksList into entry detail page

**API:**
- Added fetchBacklinks() function in knowledge.ts
- Added KnowledgeBacklink type to shared types

**Tests:**
- Comprehensive tests for BacklinksList (8 tests)
- Comprehensive tests for WikiLinkRenderer (14 tests)
- All tests passing with Vitest

**Type Safety:**
- Strict TypeScript compliance
- No 'any' types
- Proper error handling

apps/web/src/components/knowledge/EntryViewer.tsx

@@ -2,22 +2,23 @@
 
 import React from "react";
 import type { KnowledgeEntryWithTags } from "@mosaic/shared";
+import { WikiLinkRenderer } from "./WikiLinkRenderer";
 
 interface EntryViewerProps {
   entry: KnowledgeEntryWithTags;
 }
 
 /**
- * EntryViewer - Displays rendered markdown content
+ * EntryViewer - Displays rendered markdown content with wiki-link support
  */
-export function EntryViewer({ entry }: EntryViewerProps) {
+export function EntryViewer({ entry }: EntryViewerProps): React.ReactElement {
   return (
     <div className="entry-viewer">
       <div className="entry-content">
         {entry.contentHtml ? (
-          <div
+          <WikiLinkRenderer
+            html={entry.contentHtml}
             className="prose prose-sm max-w-none dark:prose-invert"
-            dangerouslySetInnerHTML={{ __html: entry.contentHtml }}
           />
         ) : (
           <div className="whitespace-pre-wrap text-gray-700 dark:text-gray-300">