fix: focus editor on new tab / tab switch / reload; prep 0.3.0

[codecancu]

Editor focus:

• New tab (+ button, File → New) now focuses the editor so typing works immediately without clicking into the text area.
• Switching to an existing tab (including via the >> overflow dropdown) focuses that pane's editor.
• After a page reload, the active editor is focused via App.focusActiveEditor(), called once the dock is initialised.

Release prep for 0.3.0:

• Bump version to 0.3.0 (package.json, package-lock.json, manifest.json, README badge + release-zip reference).
• CHANGELOG: add 0.3.0 entry (split view + the focus fixes above).

Tests: new tests/e2e/new-tab-focus.spec.ts (4 cases: + button, File → New, tab switch, reload). 640 unit + 183 e2e pass; typecheck and lint clean.

What & why

Checklist

• Signed off (DCO -s flag on all commits)
• npm run lint passes
• npm run typecheck passes
• npm test passes
• Added or updated tests for behavior changes
• New source files include // SPDX-License-Identifier: GPL-3.0-or-later