fix: refine public Paykit contact handling

[ovitrif]

Stacked on #924

Description

This follows up on the unresolved review comments from PR #924.

• Moves active contact payment context into pending state (review comment).
• Reuses shared JSON parsing (review comment).
• Centralizes the default Bolt11 invoice lifetime for regular invoice creation (review comment).
  • doesn't update lifetime of bolt11 in paykit (contrary to that PR review comment)
• Uses activity.contact() in contact activity filtering (review comment).
• Renames ActivityContactTitle.kt to ContactActivityTitle.kt (review comment).
• Refactors PublicPaykitRepoTest setup (review comment).

Preview

N/A

QA Notes

Manual Tests

• Deleted Paykit profile
• Created Paykit profile
• Added Paykit contact
• Paid LN to paykit contact
• Received LN from paykit contact

Automated Checks

• ./gradlew compileDevDebugKotlin
• ./gradlew testDevDebugUnitTest
• ./gradlew detekt

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.