Add syntax highlighting to file browser, harden capsules list

File browser:
- Add shiki-based syntax highlighting (lazy-loaded, zero initial bundle
  impact) with support for 30+ languages
- Cap highlighting at 2000 lines to avoid freezing on large files
- Pre-compute preview lines as derived state instead of re-splitting
  on every render
- Add content-visibility: auto on code lines for off-screen skip
- Remove per-line CSS transitions (unnecessary paint on 5000 elements)
- Cap row entrance animations to first 30 entries

Capsules list:
- Pause auto-refresh polling when browser tab is hidden
- Add empty state for search with no results
- Fix error state not clearing on successful refresh
- Fix action menu positioning near viewport edges
- Disable create button when no template selected

frontend/package.json

@@ -31,6 +31,7 @@
 		"@xterm/addon-fit": "^0.11.0",
 		"@xterm/addon-web-links": "^0.12.0",
 		"@xterm/xterm": "^6.0.0",
-		"chart.js": "^4.5.1"
+		"chart.js": "^4.5.1",
+		"shiki": "^4.0.2"
 	}
 }