Run the e2e suite in CI and cover the plugin's core behaviour

[frolic]

The e2e suite from #198 has never run anywhere — the repo only has manual-release.yml — and the one spec in it can't tell a working plugin from a broken one. It asserts the plugin appears in app.plugins.plugins, which Obsidian populates before onload() finishes, so it passes even when registration has failed and no file will open.

This runs the suite on pull requests and covers the behaviour an SDK upgrade is most likely to disturb, so there's a baseline to upgrade against rather than a leap of faith. It's the groundwork for #212, which bumps tldraw across a major version.

What's covered:

• creating a drawing, editing it, and reading it back from disk — nothing covered saving before
• the canvas background matching the Obsidian theme, and following a theme change
• the plugin's own canvas chrome and main menu rendering

Two decisions worth knowing about:

• Each spec creates its own drawing rather than opening a committed fixture. Nothing then depends on the schema version of a file on disk, so these specs stay valid across tldraw upgrades instead of needing new fixtures each time.
• The UI spec clicks with WebdriverIO, not element.click(). tldraw's menus open on pointer events, which a synthetic in-page click doesn't produce — it silently opens nothing and the assertion passes against an empty string.

The workflow runs under xvfb because Obsidian is an Electron app and needs a display.

Test plan

Everything here is the test. Verified locally against the plugin as it stands today — 6 tests across 4 specs, ~22s:

✓ Should appear in the plugin list
✓ writes an edit to disk and reads it back
✓ matches the Obsidian theme background
✓ follows the Obsidian theme when it changes
✓ renders the canvas chrome
✓ renders the plugin main menu

Not covered

Visual correctness — these assert elements exist, not that they look right. Also markdown view mode, embeds in reading view, and mobile.