feat: support commonAppConfig site config option

[arbrandes]

Description

Adds a new commonAppConfig optional site configuration field. When set, its values are merged into every app's config via getAppConfig() at the lowest priority, so app-specific config always takes precedence.

Work log

1. Read types.ts to understand the new commonAppConfig: AppConfig field at line 62 in OptionalSiteConfig
2. Read runtime/config/index.ts to understand how getAppConfig(), addAppConfigs(), and the appConfigs record work
3. Initially proposed merging commonAppConfig into appConfigs at write time (in addAppConfigs()), but user preferred keeping app-specific configs separate and merging at read time
4. Modified getAppConfig() to merge commonAppConfig from site config as the lowest-priority base using lodash.merge, with app-specific config on top
5. Added 4 test cases to runtime/config/index.test.ts covering all merge scenarios
6. All 18 tests pass (14 existing + 4 new)

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