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### Checklist
- [x] If applicable, unit tests
- [x] If applicable, create follow-up issues for `purchases-ios` and
hybrids

### Motivation
<!-- Why is this change required? What problem does it solve? -->
While developers' priority is always the end-user, improving the
development experience (DX) and making the daily workflow more
comfortable for developers is equally vital. By integrating convention
plugins into Version Catalogs we do unlock a lot of benefits for
development with a very few small changes:
- **Standardization**: If we already add third party's plugins via
aliases, why should we add our plugins in a different way?
- **Single source of truth**: `libs.versions.toml` is the only place to
go.
- **Less error prone**: Avoid failure builds because of a typo or
misscopying the ID from somewhere else. This also safe us some time.
- **IDE capabilites**: Enables those helpfull features that make our
life easier.
    - Code auto-completion
    - Refactoring/renaming
    - Error highlighting
    - `ctrl/cmd + click` for jumping to definition or finding usages
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**Note:** I did not create any associated issue because I did not find
any category that matches the purpose of this PR. If this is mandatory,
please do let me know and point to under what category should be
created.

### Description
Integrate convention plugins into Version Catalogs to enable typesafe
alias accessors for locally registered custom convention plugins.
#### Changes
- Declare gradle convention plugins registered in
`build-logic/convention/build.gradle.kts` on
`gradle/libs.versions.toml`.
- Replace hardcoded convention plugins IDs across all modules by the
equivalent typesafe alias accessors.
    - Before:  `id("revenuecat-android-application")`
    - After: `alias(libs.plugins.revenuecat.android.application)`
#### Testing
- Verified that all modules successfully build after switching from
hardcoded IDs to aliases.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Touches Gradle configuration across multiple modules by changing how
core convention plugins are applied, so mistakes would surface as
build/IDE resolution failures. No runtime or library logic changes.
> 
> **Overview**
> Standardizes application of internal RevenueCat convention plugins by
moving their IDs into `gradle/libs.versions.toml` and using
`alias(libs.plugins...)` accessors instead of hardcoded
`id("revenuecat-...")`.
> 
> Updates the affected app and library modules (`purchases`, `ui/*`,
`feature/*`, `examples/*`, `integration-tests`, `api-tester`) to use the
new catalog aliases for the same convention plugins.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
7b1fe62. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->