Theme selector

[SlothSoftworks]

Summary

• Add a "Theme" selector (Options -> Appearance): the original plain look ("Default MUI") or a new "SlothUI" theme matching SlothArchiver's website palette. SlothUI (dark) is now the default for a fresh install; existing installs are unaffected.
• Rework the cookie warning to explain what cookies are actually for (age-restricted/members-only content, not something to leave on for every download) and add a "Save across sessions" checkbox (off by default) so a saved cookie or cookies-from-browser choice doesn't silently outlive the app session that created it.
• Fix the bulk-add queue showing a generic "Download failed" instead of the real reason when a retry followed an earlier failure on the same item.
• Bump version to 1.0.0 and update CHANGELOG.md/README.md accordingly.

Related issues

Test plan

• npm test
• npm run lint passes
• npm run build succeeds
• Manually verified in the running app (describe the scenario below)

Notes for reviewers

• Existing users keep whatever cookie-persistence behavior they already had: the new "Save across sessions" setting only defaults to off when no cookie file exists yet; if cookies.txt is already on disk, it defaults to on (grandfathered in) so nobody's saved cookie silently disappears on upgrade.
• oxlint.config.ts now excludes **/*.test.ts/**/*.test.tsx from the anti-slop plugin -- the earlier all-error run was almost entirely as SomeMockType casts in test setup code, the same test-isolation pattern no-module-mocking was already turned off for.
• node scripts/check-release-version.mjs passes clean against the 1.0.0 bump.