BLAST Submit wizard — UX batch

Motivation

Continuation of the UI/UX improvement pass, focused on the BLAST Submit wizard. Several frictions remained: the Ctrl/⌘+Enter submit shortcut was undiscoverable, a failed submit only toasted the missing field without moving the user to it, the "Help me choose a database" panel showed a bare spinner, long example lists had no filter, and a tab refresh mid-submit could silently abandon the job.

Note: three candidates from the original list were already implemented and left untouched — the stepper sidebar is already position: sticky (blast-submit-layout.css), FASTA validation already updates reactively in QuerySection, drag-and-drop upload already exists, and the autosave "Saved Ns ago" indicator already lives in the footer. A cost/time estimate preview needs a backend estimator and is deferred.

User-facing change

• #15 Shortcut hint — the Run BLAST button now shows a Ctrl+↵ kbd badge so the existing keyboard submit is discoverable.
• #16 Scroll to first error — submitting with missing fields now scrolls the wizard to the first incomplete step (Database → Sequence → Taxonomy → Cluster in visual order) before showing the toast.
• #14 DB recommendation skeleton — the "Help me choose a database" panel renders an animated shimmer skeleton while the recommendation loads, instead of only a spinner on the button.
• #17 Example search — ExamplePicker gains a filter box (label + description) that appears only when a tool exposes more than six presets, with a "no match" hint.
• #12 Unsaved-submit guard — a beforeunload prompt now fires only while a submit is in flight, so a refresh/close won't abandon an in-progress submission. Scoped to the pending window so it never nags during editing.

Code change summary

• web/src/pages/BlastSubmit.tsx: scroll-to-first-incomplete-step in handleSubmit; beforeunload effect gated on submitMutation.isPending.
• web/src/pages/blastSubmit/BlastSubmitFooter.tsx: Ctrl+↵ kbd badge on the Run button (normal state only).
• web/src/pages/blastSubmit/DatabaseRecommendPanel.tsx: shimmer skeleton while loading.
• web/src/components/ExamplePicker.tsx: search/filter box for long preset lists.
• web/src/theme/glass.css: .blast-submit-kbd, .blast-db-reco__skeleton, .example-picker__search styles.

Validation evidence

• cd web && npm run build → clean.
• cd web && npm test -- --run submitValidation ExamplePicker blastSubmit → 217 passed.
• npx eslint on the four changed files → clean.

No backend / API / IaC changes.