refactor: idiomatic Rust improvements for performance and clarity

[niklabh]

Summary

• Remove redundant HostState::clone() in run_module — saves one full deep-clone per module load
• Consolidate double mutex lock in render_toolbar into a single lock acquisition
• Replace unconditional ctx.request_repaint() with status-aware repaint — idle state uses 200ms polling instead of burning CPU continuously
• Remove db.flush() (fsync) on every KV write — sled manages its own flush schedule, this was a major I/O bottleneck

Test plan

• Load a Wasm module and verify it runs correctly (clone fix)
• Verify back/forward buttons still work correctly (mutex fix)
• Verify UI remains responsive during module execution (repaint fix)
• Verify UI doesn't burn CPU when idle (repaint fix)
• Verify KV store data persists after writes (flush removal)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9c2afe55-38d6-465c-8189-7e85871e27d1

📥 Commits

Reviewing files that changed from the base of the PR and between 62a9f61 and 437e73d.

📒 Files selected for processing (3)

• oxide-browser/src/capabilities.rs
• oxide-browser/src/runtime.rs
• oxide-browser/src/ui.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch refactor/idiomatic-rust-improvements

📝 Coding Plan

• Generate coding plan for human review comments

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