Interactive transcript export: pre-fill the media reference from a local file's real name

[maboa]

When exporting an interactive transcript of a local media file, the media is a blob: URL with no usable path, so the export modal's "Media file or URL" field starts empty and the user has to type the reference by hand. Worse, whatever was last typed persists across opens, so a throwaway value (e.g. test) can silently end up in the exported page's <video src="…">.

Proposed improvement:

• Capture the real filename of a locally-loaded media file and pre-fill the modal field with it, so local files behave like remote URLs (which already fill from the player src) and the user rarely needs to type anything.
• Clear the field when new media is loaded, so a name typed for a previous clip can't linger and be exported by mistake.
• Capture centrally from any media file input; import inputs (JSON/SRT/VTT) are excluded.

Covered by e2e regression tests (pre-fill, stale-clear on new media load, import-input exclusion).