docs: add CONTRIBUTING guide and PR template

[jvsena42]

Summary

Adds a human-facing external-contributor guide, links it from the README, and adds a GitHub PR template so the checklist auto-populates on every PR. Structured so future code-review bots can consume it too.

• CONTRIBUTING.md (new) — covers atomic/conventional commits (with real examples + the commit types actually used here), unit tests for business logic (fakes over mocks; points at TransactionViewModelTest and fakes/FakeFlorestaRpc.kt), updating journeys/testTags for UI changes (real journey snippet + the two testTag gotchas), self-review + local detekt/lintDebug/test gates, and requiring screenshots/videos for UI changes. Opens with a note that the project is maintained solo, in free time, so reviews may take a while.
• README.md — new ## Contributing section linking the guide.
• .github/pull_request_template.md (new) — checklist mirroring the guidelines.

Docs/config only — no build impact.

Checklist

• Commits are atomic and follow conventional style
• Unit tests added/updated for business-logic changes — N/A (docs only)
• Journeys and journeys/README.md testTags updated for UI changes — N/A (no UI change)
• ./gradlew detekt, ./gradlew lintDebug, and ./gradlew test pass locally — N/A (docs only, no build impact)
• I have self-reviewed my own diff

UI changes

N/A

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