Allow optional custom name when sharing a trace

Currently, when using the "Share trace" functionality in the Perfetto UI, the system generates a permalink. However, there is no way to attach a human-readable name or description to this link at the time of creation. When sharing multiple traces or revisiting old shared links, it is difficult to distinguish them without analyzing each one.

Proposed Solution:
- Update the "Share trace" button to open a dialog to include an optional text input field (e.g., "Trace Name" or "Description").
- When generating the permalink, store this custom name alongside the trace metadata.
- When a user opens the unique permalink, display this name in the UI (e.g., in the top header bar or as the browser tab title).

If it's a simple change, I don't mind doing it myself :)

Start with the Perfetto UI's "Share trace" button and its permalink-generation flow. Trace how shared-trace metadata is stored and rendered when a unique permalink is opened; done means an optional name or description can be entered, persists with the link, and appears in the UI or browser tab title.

Written by the indexing model from the issue text.