fix(tooling): stop passing --flavor to a project with no product flavors

[koniz-dev]

build.yml offered a flavor input and passed it to flutter build for Android
and iOS. android/app/build.gradle.kts declares no productFlavors, so:

Task 'assembleDevelopmentRelease' not found in root project 'android'
[!] The android/app/build.gradle.kts file does not define any custom product
    flavors. You cannot use the --flavor option.

The working code path settled the design question

The web job in the same workflow already omitted --flavor and passed only
--dart-define=ENVIRONMENT - which is why web was the one platform that built.
That is the project's actual convention, not a workaround:

• lib/core/config/app_config.dart:33 reads ENVIRONMENT from .env or
  --dart-define.
• docs/guides/configuration.md documents env layering and dart-defines.
• No productFlavors have ever existed in the Gradle config.

So I dropped --flavor rather than inventing flavors. Adding real ones would also
require iOS schemes, which cannot be created from a script here, and would add
surface a starter does not need.

Renamed the input

flavor -> environment, described as "Value passed as
--dart-define=ENVIRONMENT". Calling it a flavor is what made the mistake
plausible in the first place. Artifact names and the concurrency group follow the
rename.

Verified by dispatching

Run	Result
32971439671 platform=android	success in 6m23s, artifact android-apks-development (32,223,960 bytes)
32971452663 platform=web	success in 1m17s, artifact web-build-development (14,226,706 bytes) - not regressed

That Android APK is the first this repository has ever produced in CI.

Deploy workflows

deploy-android.yml had --flavor "$ENV" inside its commented-out build
template; removed, so uncommenting it does not reproduce the bug.
deploy-ios.yml turned out to have no such line - I asserted it did and the
assertion caught me. Neither was dispatched: they publish to stores.

Refs #34