[Oztechan/AdTrack#132] Production AdMob App ID injection + release secrets

[mustafaozhan]

Closes #132. The go-live step for the ads epic #121 — wires the injection paths for real AdMob ids while keeping test defaults for debug/CI.

Code changes

• iOS App ID injection (the real gap): GADApplicationIdentifier was a hardcoded test literal with no injection path. Now Info.plist reads $(ADMOB_APP_ID) from Config.xcconfig (iOS test default), and fastlane build_project overrides it from the ADMOB_APP_ID_IOS env var when present (release only).
• Per-platform App IDs: the AdMob console registers the Android and iOS apps separately, so each has its own App ID. Android reads Key.ADMOB_APP_ID_ANDROID; iOS reads ADMOB_APP_ID_IOS — matching the per-platform ad-unit-id pattern. (Defaults are Google's two distinct test App IDs.)
• CI (release.yml): passes ADMOB_APP_ID_ANDROID, ADMOB_APP_ID_IOS, and the per-format ad unit id secrets as env, mirroring the OAuth vars. Each falls back to Google's test id when the secret is unset.
• Hardened common secret(): an unset GitHub secret expands to an empty env var; a blank value now falls back to the default instead of an empty id. (buildSrc getSecret, used for the Android App ID, already handled empty env.)
• docs/admob-go-live.md: the keys, where each is consumed, and how to set them.

Not in this PR (your console/secret tasks — documented)

• Create the Android + iOS apps and ad units in the AdMob console; copy the real per-platform App IDs + unit ids.
• Add them as GitHub Actions secrets (and to local secret.properties for local release builds).

No real ids are committed; everything still defaults to Google's public test ids.

Verified locally

detektAll, :androidApp:assembleDebug (manifest resolves to the Android test App ID …~3347511713 by default), :common:compileKotlinIosSimulatorArm64 all green. Info.plist (plutil), Fastfile (ruby -c), and release.yml (YAML) validated. The real-id injection is exercised by a release run once the secrets are set.

https://claude.ai/code/session_01BhF4tpJYkiDSSJVoe63Vtk

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.63%. Comparing base (f44885a) to head (d5bc1d4).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #160   +/-   ##
========================================
  Coverage    63.63%   63.63%           
========================================
  Files           69       69           
  Lines         1031     1031           
  Branches       115      115           
========================================
  Hits           656      656           
  Misses         321      321           
  Partials        54       54           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.