Finish frontend test backfill to 80% and make the coverage gate blocking

[enko]

Background

Follow-up to the code-quality foundation PR #186, which added the frontend test
infrastructure ($lib/test), the PR coverage report (danger-plugin-coverage,
report-only), and backfilled the entire API + store layer plus a set of
presentational components. At merge, frontend coverage was ~12.8% lines
(222 tests).

This issue tracks finishing the frontend backfill toward the 80% target and
flipping the coverage gate to blocking.

Scope

1. Component test backfill (the bulk)

~90 Svelte components remain untested. Priority order (highest logic/traffic first):

• Forms: friend-form.svelte (517 LOC), the remaining *-edit-form.svelte
  (email/url/date/address/social-profile/circle/relationship), collective-form,
  encounter-form, notification-channel forms.
• List/grid/detail: friend-list.svelte (653), friend-grid.svelte (419),
  friend-detail.svelte (456), collective-detail.svelte (see Refactor: split collective-detail.svelte (893 LOC) into focused pieces #187),
  encounter-list, circle views.
• Search: facet-dropdown.svelte (586), global-search.svelte (446),
  facet-chips.
• Address inputs: hierarchical-address-input (472), street-input,
  house-number-input, postal-code-input, city-select.
• Misc: badges, selects, modals, friend-multi-select, image-crop-modal.

2. Flip the coverage gate to blocking

Once the repo baseline clears 80%, change packages/danger/src/rules/coverage.ts
from report-only to failing: danger-plugin-coverage only posts a table (it never
calls fail()), so add an explicit fail() when changed files fall below the
80% threshold. See the note in that rule.

3. (Optional) Backend coverage in CI

Backend coverage is currently not generated in CI because v8 instrumentation
slows the better-auth integration tests past their 5s timeout. Options: raise
testTimeout under coverage, or run coverage only over the unit suite. Then add
apps/backend/coverage/clover.xml to the danger rule (the path is already listed).

Established patterns (from PR #186)

• Helpers in apps/frontend/src/lib/test/ — render, stubFetch/jsonResponse,
  createUpdateRecorder, and typed fixtures (aFriend, aCollective, ...).
• Mock modules with shared state via vi.hoisted (see stores/auth.test.ts,
  stores/friends.test.ts).
• For i18n components, mock $lib/i18n so $i18n.t echoes the key, and match
  labels as substrings — required fields append " *" (see
  subresources/phone-edit-form.test.ts).
• Subresource rows render twice (mobile + desktop) — use getAllBy*.
• Run the full local gate before pushing: vitest + biome check +
  aube --filter @freundebuch/frontend run type-check.

Acceptance criteria

• Frontend coverage ≥ 80% (lines) per the danger report.
• Coverage gate flipped to blocking.
• aube type-check, aube run check, and the full test suite stay green.