fix(desktop): refresh loopback preview assets

[oshtz]

Summary

• reload loopback previews without using cached local HTML, CSS, JavaScript, or assets
• preserve cookies and site storage while refreshing local preview content
• keep normal browser caching for public HTTPS previews
• publish the fix as v0.2.26 with a summary-only maintenance entry

Verification

• git diff --check
• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo clippy --manifest-path apps/desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• Tauri cargo test --lib with the repository's Windows manifest flags (68 passed)
• pnpm -C apps/desktop verify
• pnpm -C apps/desktop verify:tauri
• pnpm -C apps/site build
• focused WebView2 cache/persistence assertions completed against an isolated-identity Tauri binary; the local harness later reported an unrelated Codex startup 502 while another Milim instance remained open