server: open file URI ranges

[aikins01]

Amp.nvim now honors line fragments on file:// URIs from the CLI, so clicking an Amp file or range link opens the file at the requested location instead of treating the fragment as part of the path.

• parses #L10, #L10C3, #L10:3, and #L10-L12 fragments before converting the URI to a filename
• jumps to the requested line/column and centers it in the window
• briefly highlights the target line range with an extmark that clears per buffer