chore(deps): upgrade leptos 0.7 -> 0.8 (resolves wasm-streams dup)

[intendednull]

Summary

Bumps leptos from 0.7.8 to 0.8.19 in crates/web/Cargo.toml.

Why

CI Browser tests have been silently broken for weeks. Two transitive crates pulled different wasm-streams versions, causing a duplicate-symbol link error in the wasm test binary:

• wasm-streams 0.4.2 <- server_fn 0.7.8 <- leptos 0.7.8
• wasm-streams 0.5.0 <- reqwest 0.13.2 <- iroh-relay

The link error was masked by a missing pipefail in CI's bash -e {0} shell (a separate fix on ui/phase-3a-composer adds pipefail, surfacing the failure). We can't easily drop the iroh side, so the fix is to bump Leptos: server_fn 0.8.x uses wasm-streams 0.5, aligning both sides.

After this lands, ui/phase-3a-composer will rebase on main and its 39 browser tests will run on CI for the first time.

Breaking changes from 0.7 -> 0.8

None applied to this crate. The Leptos 0.8 release notes flag a few areas (LocalResource no longer wrapping in SendWrapper, server-fn error type changes, axum 0.8) but none affected our CSR-only web crate. cargo build, cargo check --target wasm32-unknown-unknown, cargo clippy, and cargo test all pass with zero source changes — only crates/web/Cargo.toml and Cargo.lock differ.

Verification

wasm-streams deduplication confirmed:

$ cargo tree -i wasm-streams --target wasm32-unknown-unknown -p willow-web
wasm-streams v0.5.0
├── reqwest v0.13.2
│   ├── iroh v0.98.1
│   │   ├── iroh-gossip v0.98.0
│   │   │   └── willow-network
│   │   │       ├── willow-client
│   │   │       │   └── willow-web
│   │   │       └── willow-web
│   │   └── willow-network (*)
│   └── iroh-relay v0.98.0
│       └── iroh v0.98.1 (*)
└── server_fn v0.8.12

Single version 0.5.0, sourced by both reqwest and server_fn.

Before this PR: two versions (0.4.2 and 0.5.0) pulled in for the wasm32 target.

Test plan

• cargo build --workspace
• cargo check --target wasm32-unknown-unknown -p willow-web
• cargo check --target wasm32-unknown-unknown --workspace --exclude willow-relay --exclude willow-worker --exclude willow-replay --exclude willow-storage --exclude willow-agent
• cargo check --tests --target wasm32-unknown-unknown -p willow-web (test binary compiles)
• just clippy — zero warnings
• just test — all native tests pass
• just fmt — no diff
• cargo tree -i wasm-streams --target wasm32-unknown-unknown -p willow-web reports a single version
• CI Browser tests job goes green (the load-bearing verification — only observable on CI)

🤖 Generated with Claude Code

[intendednull]

e2e is still failing, plz fix