FN-6130: make compound engineering theme colors dark-mode aware

Align compound engineering dashboard colors with the active theme tokens.

- replace hardcoded and legacy compound engineering CSS colors with dashboard theme tokens
- switch dimmed text treatments from opacity to explicit theme-aware text tokens for light and dark mode
- add regression coverage for token usage in compound engineering styles
- keep branch authority checks compatible with the optional base SHA argument

Files changed:
 packages/engine/src/branch-conflicts.ts            |   1 +
 .../src/dashboard/CompoundEngineeringView.css      | 106 +++++++++++----------
 .../src/dashboard/__tests__/theme-tokens.test.ts   |  97 +++++++++++++++++++
 3 files changed, 153 insertions(+), 51 deletions(-)

Fusion-Task-Id: FN-6130

Fusion-Task-Lineage: 00adc217-0649-4b4f-9b9b-92c551f02926

packages/engine/src/branch-conflicts.ts

@@ -377,6 +377,7 @@ export async function isBranchAuthoritativeForTask(
   repoDir: string,
   branch: string,
   taskId: string,
+  _baseSha?: string,
 ): Promise<{ ok: true } | { ok: false; reason: string }> {
   try {
     await revParse(repoDir, `refs/heads/${branch}`);