rebrand oauth success page to match kernel.sh (#151)

## Summary

Updates the CLI OAuth success page (`pkg/auth/success.html`) to match
the new kernel.sh brand:

- light beige background (#f2f0e7) instead of dark (#2C2C2C)
- Inter Light (300) font instead of Commit Mono
- hard corners throughout (0px border-radius)
- 0.5px solid charcoal borders
- kernel green (#81b300) checkmark instead of purple (#AC86F9)
- inline krnl. wordmark SVG
- lowercase text-transform
- removes ~100KB base64 background image, bringing file from 105KB → 9KB

Matches the styling applied to the MCP server OAuth pages in
kernel/kernel-mcp-server#99.

## Test plan

- [ ] Run `kernel auth login` and complete the OAuth flow
- [ ] Verify the success page shows with beige background, krnl.
wordmark, and green checkmark
- [ ] Verify the page renders correctly on different screen sizes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk visual-only change to a static OAuth success HTML page; no
authentication flow or data handling logic is modified.
> 
> **Overview**
> Rebrands the CLI OAuth success page (`pkg/auth/success.html`) to match
kernel.sh with a new light theme, Inter typography, lowercase styling,
and updated layout.
> 
> Removes Tailwind and the large embedded base64 background image, and
replaces the header and iconography with an inline Kernel wordmark SVG
and a green checkmark.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
ab6fbde. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: masnwilliams