feature: add a persistent 'export all notes' backup action

[tech-adrian]

Area
frontend/src/lib/notes.ts, frontend/src/app/{withdraw,history,compliance}/page.tsx

Problem
The only bulk-backup affordance in the app is downloadBackup() on the deposit page, and it's scoped to sessionNotes — notes created in the current deposit call only, not the full set of notes held in localStorage. A user who imported notes via NoteImport, received a note from someone else, or simply didn't click "Download" right after depositing has no way to export a single backup file covering everything they currently hold, even though Withdraw/History/Compliance all read and display the full note list. This is a distinct gap from QR export/import (#12, a new encoding format for individual notes) and CSV/JSON history export (#13, transaction/report records) — this is about bulk-exporting the actual spendable notes a user already has.

Proposed change
Add a persistent "Download all notes" action (reusing the existing serializeNote format) available from Withdraw, History, and/or Compliance — not just the ephemeral post-deposit panel — so a user can back up their full note set at any time.

Acceptance Criteria

• A "Download all notes" action exists outside the post-deposit flow, covering every note in storage
• Exported format matches the existing single-note serialization so it round-trips through NoteImport
• Relevant tests (just test, pnpm test, cargo test, or nargo test as applicable) pass locally and in CI
• A screenshot or short screen recording is attached to the PR showing the change working (exporting all notes and re-importing the file) — required before this can be merged.

Open your PR against the dev branch, not main. All active development merges into dev.

Discuss this issue / coordinate work: join the DShield contributor Telegram group: https://t.me/+SiGHH24No9U2MDJk

[soloking1412]

I work on frontend and note/state serialization logic regularly across wallet and payment UIs, so reusing an existing serialization format for a new export surface without breaking round-trip compatibility is a straightforward extension for me. For this I'd read the full note set straight from localStorage rather than sessionNotes, reuse serializeNote as-is so the output matches the exact format NoteImport already expects, and add a persistent "Download all notes" button on Withdraw, History, and Compliance, wherever the full list is already rendered rather than building a new data path. I'd verify the round-trip manually, export the full set, clear storage, re-import, confirm notes match, before recording the required screenshot. I'd run the test suite locally and add a small test asserting the export covers every stored note, not just session-scoped ones, since that's the actual bug this issue is closing. Happy to take this on.

[grantfox-oss]

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Next steps:

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[grantfox-oss]

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@soloking1412's PR #73 was approved and merged by @tech-adrian.

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