feat(tui): E keybind on Question Dashboard exports bibliography

[gerchowl]

Refs #135 (sub-feature B of 3)

Summary

• Adds E keybind on Overlay::QuestionDashboard that opens a path-prompt overlay pre-filled with a slugified default (what-is-the-role-of-attention.bib); Enter writes the .bib + .scitadel-bib.lock sidecar through the same scitadel_export::write_snapshot helper the CLI now uses.
• Lifts the snapshot write path (SnapshotFormat, write_snapshot, default_filename_for_format, sidecar_path_for, slugify) into scitadel-export so CLI + TUI emit byte-identical artifacts; refactors bib_snapshot in scitadel-cli to call through the new helper.
• Status-bar toast on success (exported: paper.bib (12 entries) for ~30 frames) and error (export failed: <err> for ~60 frames). Reuses the existing per-frame counter pattern (feat(tui): light mode + auto-detect (theme iter 2) #137) — no new toast subsystem.

What landed

• crates/scitadel-export/src/slug.rs — kebab-slug helper with diacritic folding, length cap, fallback for all-non-alpha input.
• crates/scitadel-export/src/snapshot.rs — write_snapshot(...) pure write helper + SnapshotFormat enum.
• crates/scitadel-tui/src/views/bib_export_prompt.rs — single-buffer path prompt with slugified default and centred modal renderer.
• crates/scitadel-tui/src/app.rs — E handler, layered overlay key dispatch (prompt eats keys before list controls), StatusToast slot, status-bar wiring, end-to-end tests.
• tests/vhs/tui-export-keybind.tape — VHS tape (uncrendered locally; vhs/ttyd not present).

Test plan

• cargo test --workspace --exclude scitadel-tui — all green
• cargo test -p scitadel-tui — 42 tests pass (4 new app::tests + 5 view tests + 9 slug/snapshot tests)
• just lint — fmt + clippy clean
• VHS tape rendered in CI (vhs/ttyd not on this machine; tape committed for CI/PR-review render)
• Manual smoke: scitadel tui → Questions tab → Enter → E → Enter → confirm paper.bib + paper.bib.scitadel-bib.lock appear, toast shows entry count