Fix CI failures: Update Ubuntu 24.04 package names and resolve clippy errors

[Copilot]

All CI jobs were failing due to deprecated package names for Ubuntu 24.04 and code quality issues caught by clippy's -D warnings flag.

Package Dependencies

• Updated GTK/WebKit packages for Ubuntu 24.04 compatibility:
  • libwebkit2gtk-4.0-dev → libwebkit2gtk-4.1-dev
  • libjavascriptcoregtk-4.0-dev → libjavascriptcoregtk-4.1-dev
• Fixed invalid Rust edition in Cargo.toml: "2024" → "2021"

Code Quality Fixes

• FFI safety: Marked functions dereferencing raw pointers as unsafe

  // Before
  pub extern "C" fn nexus_initialize_database(db_path: *const c_char) -> *mut DbConnection {
      let c_str = unsafe { CStr::from_ptr(db_path) };
  
  // After
  pub unsafe extern "C" fn nexus_initialize_database(db_path: *const c_char) -> *mut DbConnection {
      let c_str = CStr::from_ptr(db_path);

• CRDT: Fixed non-canonical PartialOrd to delegate to Ord::cmp
• Closures: Replaced redundant closures with function references (|e| fn(e) → fn)
• Pattern matching: Converted single-arm match to if-let
• Unused code: Prefixed unused variables/fields with underscore
• Formatting: Applied cargo fmt for import ordering

Original prompt

Fix CI workflows. None of the CI tasks are returns success.

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