fix(frontend): guard stream progress bar against BigInt divide-by-zero (#550)

[dmystical-coder]

Description

The stream-detail progress bar crashed when a stream's depositedAmount was "0". It computed (withdrawn * 100n) / deposited with BigInt operands, and BigInt division by zero throws RangeError, crashing the page render. This guards that division.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🔧 Refactoring (no functional changes)
• ⚡ Performance improvement
• 🧪 Test addition or update

Related Issues

Closes #550

Changes Made

• frontend/src/utils/amount.ts: add a pure, exported streamProgressPercent(withdrawn, deposited) helper. Returns 0 when nothing is deposited (100 if anything was withdrawn) and clamps the result to [0, 100].
• frontend/src/app/streams/[id]/page.tsx: use the helper instead of the inline (withdrawn * 100n) / deposited expression that threw.
• frontend/src/__tests__/utils.test.ts: add tests for the helper, including a regression test asserting the zero-deposit case does not throw.
• package-lock.json: reconcile the root lockfile with package.json (rollup platform deps 4.59.0 → 4.60.2; @rollup/rollup-linux-x64-gnu@4.60.2 was missing). This unblocks npm ci for all CI jobs — it was failing repo-wide on main, not just on this branch.
• frontend/package.json + lockfile: add the missing @testing-library/dom peer dependency required by @testing-library/react@16. Without it the component/hook test suites could not load (Cannot find module '@testing-library/dom') once npm ci was unblocked. (Also clears 4 pre-existing tsc errors for screen/fireEvent.)
• frontend/src/hooks/useModalDialog.ts: bind the modal Escape/focus-trap keydown handler to window instead of document so it also catches window-level keydown events. Fixes the previously-failing CancelConfirmModal Escape-to-close test.

Note: the lockfile, missing peer dep, and modal-handler items above were all pre-existing breakage unrelated to issue #550 — CI was red across the board on main. They are bundled here (at the maintainer-assignee's direction) so this PR's CI goes fully green and the #550 fix is actually validated end-to-end.

Testing

• npx vitest run src/__tests__/utils.test.ts → 49 passed (45 existing + 4 new)
• npm run lint → 0 errors (no new warnings in changed files)
• npm run build → could not be completed in my local sandbox because app/layout.tsx fetches Google Fonts (IBM Plex Mono, Sora) at build time and outbound network is blocked here; this is unrelated to the change and is covered by CI.
• npm ci --include=optional --dry-run → passes after the lockfile reconcile (previously failed; this is what was breaking CI on main).
• npm test (full vitest suite) → 74/74 pass after adding @testing-library/dom and the modal-handler fix (previously 2 suites failed to load).
• npx tsc --noEmit → 0 errors (the 4 pre-existing screen/fireEvent errors are resolved by the peer dep).
• npm run lint → 0 errors (5 pre-existing warnings in untouched files).

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed (verified the exact arithmetic that threw via unit tests; the page is a heavy client component and the repo's @testing-library/react render harness is currently broken, so a full render test was not viable)

Test Steps

1. Open a stream-detail page for a stream with depositedAmount: "0".
2. Before: the page crashed with RangeError: Division by zero.
3. After: the page renders; the progress bar shows 0% (or 100% if anything was withdrawn).

Screenshots/Demo

No screenshot attached: the change is a one-line guard with no visual restyling, and the app cannot be built/run in my sandbox (Google Fonts fetch is network-blocked, see Testing). The bar fills ${progressPercent}% exactly as before for normal streams; the only behavioral change is that a zero-deposit stream now renders 0% instead of crashing.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (not applicable)
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes
• I have checked for breaking changes (none)

Additional Notes

Commit follows the conventional type(scope): description format and the branch follows the fix/short-description convention per CONTRIBUTING.md.

---

CI note: pre-existing backend failures (tracked separately)

Repairing the root lockfile unblocked npm ci, which surfaced 22 pre-existing backend integration-test failures (all blanket 401) in the Backend CI / Backend npm test jobs. These are unrelated to this PR — it changes no backend code or backend dependencies — and they were red on main already (just masked by the earlier npm ci failure). Tracked in #608. All frontend and #550-related checks on this PR are green.