fix(spa): 404 for missing assets; persist selected project

[ajianaz]

What

• spa.rs: asset-like paths (final segment has an extension, or under _app/) that are not embedded now return 404. Extension-less client routes still fall back to index.html.
• web: selected project persists across Issues/Transactions/Release Health/Rules via localStorage (lib/project-selection.ts). Resolution order: valid ?project=, stored value, first project.
• trapfalld/build.rs: emits a cargo warning when web/build/_app is missing (a fresh clone only tracks a placeholder index.html, so the dashboard is blank without npm run build). Relates to bug(ui): dashboard renders blank on fresh clone — web/build assets not in git but embedded at compile time #307 (not closed here).

Why

Missing JS/CSS returned index.html with 200, hiding build problems and confusing browsers. Project choice was lost when navigating between pages.

Testing

• Rust tests written first (failed with 200 vs 404), now pass; cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test --workspace all pass.
• web: npm ci, npm run check (0 errors), npm test (new project-selection.test.ts), npm run build all pass.

Closes #310
Closes #311

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