[Desktop, Windows] File links outside the session folder get no file menu, and no pane can show them

**What's wrong**
In the Code tab on Windows, a markdown link only gets the file context menu (Attach as context, Copy path, Open in) when its target is a path relative to the session folder with no `:line` suffix. These all fall back to the plain message menu, so there is no "Open in VS Code":
- absolute target: `[x](C:/Users/me/.claude/CLAUDE.md)`
- absolute target inside the session folder: `[x](D:/proj/CLAUDE.md)`
- relative target with a line: `[x](CLAUDE.md:5)`
- relative target using `..`
- `file:///` and `vscode://` targets

Only `[x](CLAUDE.md)` works. Files the agent edits outside the session folder (for example `~/.claude`) therefore cannot be opened from chat at all, and the file pane and diff pane refuse them too ("could not be resolved inside this session's working directory or granted folders"; diff pane shows "No changes to show").

**Expected**
- Any link that resolves to an existing file gets the file menu, including absolute paths and `:line`, and Open in passes the line to the editor.
- The diff pane can show changes in granted or additional directories that are git repos (desktop counterpart of #92108).
- Optional: a default-editor setting so one click opens the external editor (#54027, #89598 were closed as not planned).

**Steps**
1. Windows 11, Claude desktop, Code tab, session in `D:\proj`.
2. Have the assistant output the link forms above.
3. Right-click each.

**Environment**
- Claude desktop 2.110.0.0 (MSIX)
- Claude Code 2.1.263
- Windows 11 Pro 10.0.26200

**Related:** #61443, #72423, #68559, #92108

Start with the Code tab's markdown-link handling, then trace the file pane and diff pane resolution paths described in the report. Reproduce the listed absolute, line-suffixed, parent-directory, file:///, and vscode:// links on Windows; done means existing files receive the file menu, Open in preserves the line, and eligible external git repositories render in the diff pane.

Written by the indexing model from the issue text.