fix: migrate to nostr-sdk 0.45

[akiomik]

Summary

• migrate nostui to the nostr-sdk 0.45 API included by Dependabot PR build(deps): bump nostr-sdk from 0.44.1 to 0.45.1 #501
• preserve read-only npub mode by keeping the public key separate from optional signing keys
• replace removed signer, tag, subscription, event-builder, and root re-export APIs

Root cause

nostr-sdk 0.45 removes the client signer abstraction, standardized tag APIs, several event-builder helpers, and root-level type re-exports used by nostui.

Validation

• cargo fmt --all --check
• cargo test --all-features
• cargo llvm-cov --all-features --workspace --lcov --output-path /private/tmp/nostui-lcov.info
• cargo clippy --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS='-D warnings' cargo doc --no-deps --document-private-items --all-features --workspace --examples
• typos

Supersedes #501.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.91%. Comparing base (e331bf5) to head (e6b4eff).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/infrastructure/subscription/nostr.rs	62.85%	39 Missing ⚠️
src/runtime.rs	72.41%	8 Missing ⚠️
src/main.rs	0.00%	4 Missing ⚠️
src/domain/nostr/nip10.rs	97.29%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #502      +/-   ##
==========================================
+ Coverage   88.82%   88.91%   +0.09%     
==========================================
  Files          42       41       -1     
  Lines        6909     6921      +12     
==========================================
+ Hits         6137     6154      +17     
+ Misses        772      767       -5     

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