fix: bring example-client-app to holochain 0.6.1-rc.7 so it builds again

[zippy]

Why

The Build apps (example-client-app) CI job has failed on every PR since the hc 0.6 update landed in December 2025. The app's zome workspace was never brought along to 0.6: it still pinned hdk 0.6.0 / hdi 0.7.0 / holochain_serialized_bytes 0.0.56 (which pins serde =1.0.219), while the path-dep chain through crates/tauri-plugin-client → runtime-types-ffi pulls holochain_conductor_api 0.6.1-rc.7, which pins hsb =0.0.57 (serde =1.0.228). Two exact serde pins in one graph → failed to select a version for serde, unconditionally. The app's Cargo.lock was still from the 0.5 era (conductor_api 0.5.4), so every build re-resolved and hit the wall.

What

All changes under apps/example-client-app/:

• Cargo.toml: pin hdk =0.6.1-rc.5 / hdi =0.7.1-rc.5 / holochain_serialized_bytes 0.0.57 — the exact versions the parent workspace's Cargo.lock resolves for holochain 0.6.1-rc.7, so the zome wasm matches the conductor the service ships.
• Cargo.lock: regenerated, then every floating holochain-family crate (zome_types, integrity_types, sqlite, holo_hash, mr_bundle, nonce, timestamp, …) aligned to the parent lock's versions — zero holochain-family drift between the two lockfiles.
• .cargo/config.toml (new): getrandom_backend=\"custom\" rustflag for wasm32 — hdk/hdi ≥ 0.6.1 depend on getrandom 0.3, which refuses wasm32-unknown-unknown without a configured backend; the hdk provides the custom one (conductor random_bytes host fn). Same config hc-scaffold generates for new hApps.
• .gitignore: the blanket /.cargo/ ignore would have silently excluded that config; narrowed to /.cargo/* + !/.cargo/config.toml.
• src-tauri/src/main.rs: the bin called tauri_app_lib::run() but the lib target is forum_tauri_lib — a rename that never propagated, invisible to CI because tauri android build only compiles the lib. Also drops the WEBKIT_DISABLE_COMPOSITING_MODE workaround (a no-op since webkitgtk 2.46).

Base branch

Stacked on #13 (chore/self-contained-dev-shell): the fresh resolution pulls edition2024 manifests that need cargo ≥ 1.85, which #13's rust 1.88 toolchain provides (main's stale rust-toolchain.toml said 1.81). GitHub will retarget this PR to main automatically when #13 merges.

Verification (in the #13 dev shell)

• cargo build --release --target wasm32-unknown-unknown --workspace --exclude forum-tauri — zomes build
• hc app pack workdir --recursive — forum.happ produced
• cargo ndk -t x86_64 check -p forum-tauri — exit 0 (the compile path the failing CI job exercises)

Note: a host-side cargo check -p forum-tauri can never pass by design — the client plugin is #![cfg(mobile)].