test(renderer): run every Electron suite from one command

The Electron suites were only reachable one npm script at a time, so a new
one was covered by whoever remembered it existed. test:electron:all builds
once and runs all five, reporting a single summary and a non-zero exit.

The runner lives under tests/ rather than app/scripts/, which .gitignore
excludes entirely.

The image row spacing assertion also stops hard-coding the current gap --
it had 0.59px of headroom -- and calibrates against the spacing the rest of
the timeline is using.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/tests/run-electron-suites.mjs

@@ -0,0 +1,32 @@
+// Runs every Electron regression suite against one build and reports a single
+// summary. Each suite already prints its own PASS/FAIL lines; this exists so a
+// new suite is covered by one command instead of only being run by whoever
+// remembers it is there.
+
+import { spawnSync } from 'node:child_process'
+import path from 'node:path'
+import { fileURLToPath } from 'node:url'
+import electron from 'electron'
+
+const appRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..')
+const suites = [
+  'electron-concurrency.mjs',
+  'electron-session-images.mjs',
+  'electron-session-virtualization.mjs',
+  'electron-session-reader-state.mjs',
+  'electron-file-references.mjs',
+]
+
+const failed = []
+for (const suite of suites) {
+  console.log(`\n=== ${suite} ===`)
+  const result = spawnSync(electron, ['--no-sandbox', path.join('tests', suite)], {
+    cwd: appRoot,
+    stdio: 'inherit',
+  })
+  if (result.status !== 0) failed.push(`${suite} (exit ${result.status ?? 'signal'})`)
+}
+
+console.log(`\n${suites.length - failed.length}/${suites.length} Electron suites passed`)
+for (const failure of failed) console.error(`FAILED: ${failure}`)
+process.exit(failed.length ? 1 : 0)