Allow optional custom name when sharing a trace

[rasikanavarange]

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 :)

[primiano]

I'd be positive about this change, as long as we can fit it into the existing TraceInfo.traceTitle (or some existing field) rather than adding yet another state field (we already have too many between title, file name, url... I would not add one more)

Essentially: i'd be positive as long as it fits well in trace_source.ts and the rest of the code.