fix(dashboard): preserve secret env values on config save

[AbdullahM07]

Summary

• keep persisted secret values masked and track their server-side identity in the configuration editor
• save existing-project env changes through the partial merge API before reporting success or deploying
• omit existing-project env values from build requests while preserving the initial-deploy flow for new projects
• render unrevealable masked values as placeholders so typing replaces the secret instead of appending to the mask

Tests

• bun --cwd apps/dashboard test (110 files, 1138 tests)
• bun run --cwd apps/dashboard lint

Fixes #800