Fix button fallback regression, add link kind, update usage in dark-mode auth templates

[burieberry]

• Fix boxel button fallback color regression (Previously we were relying on fallbacks for differentiating the themeless button. Now we use theme.css for the defaults)
• Add button link kind (no button chrome)
• Update usage in dark-mode host auth templates

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

• Boxel UI staging preview

Host Test Results

    1 files  ±0      1 suites  ±0   3h 3m 33s ⏱️ + 4m 9s
3 491 tests ±0  3 476 ✅ ±0  15 💤 ±0  0 ❌ ±0 
3 510 runs  ±0  3 495 ✅ ±0  15 💤 ±0  0 ❌ ±0 

Results for commit 91b243d. ± Comparison against earlier commit 26fda42.

Realm Server Test Results

    1 files  ±0      1 suites  ±0   11m 56s ⏱️ +6s
1 830 tests +4  1 830 ✅ +4  0 💤 ±0  0 ❌ ±0 
1 909 runs  +4  1 909 ✅ +4  0 💤 ±0  0 ❌ ±0 

Results for commit 91b243d. ± Comparison against earlier commit 26fda42.

[chatgpt-codex-connector]

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