Fix WASM compilation and add cross-platform time support

[Gozala]

Description

This PR fixes WASM compilation errors and adds proper cross-platform time
support using web-time.

Problem

The crate had WASM-specific code using js_sys::Date, wasm_bindgen, and
JsError but:

1. These dependencies weren't declared in Cargo.toml
2. std::time::SystemTime::now() panics on WASM with "time not implemented
   on this platform

Changes

Add missing WASM dependencies (ucan/Cargo.toml)

• Added js-sys, wasm-bindgen, and web-time as wasm32 target dependencies

Fix cross-platform time support (ucan/src/time/timestamp.rs,
ucan/src/time/error.rs)

• Use web-time crate for SystemTime, Duration, and UNIX_EPOCH on WASM
• Use std::time on native platforms

Fix WASM Timestamp methods (ucan/src/time/timestamp.rs)

• Removed #[wasm_bindgen] attribute from the WASM impl block (was causing
  trait bound errors)
• Fixed self.time → self.0 bug in to_date()
• Changed from_date() return type from Result<Timestamp, JsError> to
  Result<Timestamp, OutOfRangeError>