Preserve unchanged secret env values

[eetoc]

Summary

Fixes #25 by allowing existing masked global Secret environment variables to be saved without re-entering their values.

Details

• Treat an empty incoming Secret value as "preserve the existing Secret" only when that Secret already exists in global env storage.
• Reject new empty Secret values with so empty strings do not become valid Secret payloads.
• Keep deletion semantics unchanged: removing an env item from the submitted list deletes it.
• Update the settings page so unknown existing Secrets can be saved, while newly entered empty Secrets are still blocked.
• Reload global env items after save so preserved Secrets return to the masked/unknown state in the UI.

Validation

• ok agent-compose/pkg/agentcompose (cached)

check:ui
vite build --config frontend/vite.config.ts

4:42:22 PM [vite-plugin-svelte] no Svelte config found at /private/tmp/agent-compose-fix-secret/frontend - using default configuration.
vite v7.3.2 building client environment for production...
transforming...
✓ 273 modules transformed.
rendering chunks...
computing gzip size...
../dist-ui/index.html 0.37 kB │ gzip: 0.26 kB
../dist-ui/assets/index-DzucqM_C.css 112.91 kB │ gzip: 18.82 kB
../dist-ui/assets/index-420bYWVw.js 703.94 kB │ gzip: 189.13 kB
✓ built in 5.05s

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