feat: add Files tab for inline project file browsing

[dcieslak19973]

Closes #3443

Summary

• Adds a Files tab to the right sidebar, feature-gated behind a new FILE_BROWSER experiment flag (disabled by default, user-toggleable in Settings → Experiments)
• New listProjectFiles and readProjectFile IPC endpoints on the backend with path-traversal protection and binary file detection
• File tree shows directories first, then files alphabetically; clicking a directory navigates into it with breadcrumb navigation
• Markdown files rendered via the existing MarkdownRenderer; all other files syntax-highlighted via HighlightedCode (Shiki) — HTML is shown as highlighted source, never live-rendered (XSS prevention)
• Breadcrumb uses pathDirname walk rather than string splitting — correct on both Windows (backslash) and POSIX paths
• Stale request cancellation: AbortController cleanup in useEffect for directory listing; load-ID counter for file reads

Design decisions

• Follows the existing Agent Browser / Portable Desktop featureFlag gating pattern exactly
• Reuses MarkdownRenderer and HighlightedCode rather than adding new renderers
• Separate listProjectFiles endpoint (returns files + dirs) rather than reusing the existing listDirectory (dirs only) to avoid breaking DirectoryPickerModal
• 512 KB read cap with truncation warning; binary files (null-byte detection) rejected with a user-visible error

Test plan

• Enable "File Browser" in Settings → Experiments
• Files tab appears in right sidebar
• Can browse directories and navigate with breadcrumbs
• Markdown files render formatted
• Code files render with syntax highlighting and line numbers
• HTML files show as highlighted source (not rendered)
• Binary files show an error message
• Disabling the experiment removes the tab

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