feat(contract): add emergency pause mechanism (#672)

[nupedev]

Summary

Closes #672

Adds an emergency pause mechanism to the SolarGrid Soroban contract and exposes the state in the frontend.

Changes

• Adds admin-only pause(), unpause(), and is_paused() contract methods.
• Blocks make_payment and register_meter while paused.
• Keeps update_usage and read-only contract methods available during a pause.
• Automatically clears a pause at the 48-hour maximum and emits contract_unpaused.
• Emits contract_paused and contract_unpaused lifecycle events.
• Adds a global frontend ContractPauseBanner that polls the contract pause view and uses the existing English/French i18n catalogs.
• Adds the rlib target needed for isolated contract integration tests and fixes the existing apply_usage balance lookup that was preventing usage updates from compiling.

Verification

• cargo test -p solar_grid --test emergency_pause — 2 passed.
• git diff --check — passed.

The complete upstream contract unit-test target currently has unrelated pre-existing Symbol/String test-harness compilation errors; the new integration suite passes independently against the production contract library.

[drips-wave]

@nupedev Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits