Release the browser extension as 0.3.0

[rahulmahadik]

Browser extension 0.3.0.

The extension has no source changes of its own. It bundles @asksql/react, which landed in #19, so
this release carries those user-visible changes into the side panel:

• The model's output is shown as it streams, so a local model no longer looks frozen on "Writing SQL"
  for a minute with nothing moving.
• Copy controls on explanations, schema answers, the query plan and the result grid.
• No red error while an answer is still being written. A question answered from the schema marked the
  turn failed for the whole second round trip, telling the reader to rephrase a question that was
  being answered.
• The transcript stays where you put it instead of scrolling to the bottom on every update.
• Truncated cells show their full value on hover; Export CSV reports whether it worked.
• Stopping a turn is called Cancel, matching the other two surfaces.

package.json and manifest.json both carry the version and are set to 0.3.0. The release workflow
checks the tag against package.json, and the changelog guard reads the same file, so the two must
agree.