Add PR viewer UI app with diff and details tabs

[olaservo]

Summary

Adds a new MCP App UI for the pull_request_read tool that provides a visual PR viewer with tabbed Diff and Details views.

Why

The existing UI apps (get-me, issue-write, pr-write) cover user info and write operations, but there's no rich UI for reading pull request data. This adds a visual diff viewer and PR details view attached to the pull_request_read tool.

What changed

• Added ui/src/apps/pr-read/ with vanilla TypeScript app (App.ts, diff-parser.ts, diff-renderer.ts, pr-details-renderer.ts, styles.css, index.html)
• Diff view supports unified and split modes with file collapsing, progressive rendering, and fullscreen toggle
• Details view renders PR state, metadata, labels, branches, stats, and body
• Method-driven tab selection with ability to switch views via tabs (each tab triggers a callTool call)
• Added PullRequestReadUIResourceURI constant and Meta field to pull_request_read tool in pullrequests.go
• Registered pr_read_ui resource in ui_resources.go
• Made Vite React plugin conditional in vite.config.ts for non-React apps
• Added build:pr-read script to ui/package.json
• Updated pull_request_read.snap toolsnap

MCP impact

• Tool schema or behavior changed — Added _meta.ui with resourceUri to the existing pull_request_read tool. No schema or behavioral changes to the tool itself; the UI metadata is stripped when InsidersMode is off.

Prompts tested (tool changes only)

• "Show me the diff for PR Issue: Fix incorrect VS Code installation URL (both buttons point to Insiders) #123 in owner/repo"
• "Get details for pull request Bump go-github to v72.0.0 #456 in owner/repo"

Security / limits

• No security or limits impact — Read-only UI rendering of existing tool data. No new API calls or auth changes.

Tool renaming

• I am not renaming tools as part of this PR

Lint & tests

• Tested locally with ./script/test — All Go tests pass including updated toolsnap
• Linted locally with ./script/lint

Docs

• Not needed