Tests: expand Compose screen test coverage

[patrickrb]

Follow-up from #27 / PR #58.

PR #58 added FilterChipsScreenTest as the template for screen-level
testing — it runs under Robolectric + createComposeRule() on the JVM,
so no emulator is needed. The remaining major screens are uncovered.

Screens to cover

• WaterfallScreen — spectrum render + frequency picker interactions
• DecodeScreen — decoded-message list, filter chips wired to
  DecodedRow, weak-signal asterisk handling
• LogbookScreen — list render, edit/delete row actions, sync chip
  state, QRZ avatar load (with the image fetch mocked or stubbed)
• MapScreen — base render + PSK Reporter overlay toggle (mock the
  client via the baseUrl seam already in place)
• SettingsScreen — radio model picker, control-mode toggle, QRZ
  test-connection flow

Pattern

Mirror FilterChipsScreenTest:

@RunWith(AndroidJUnit4::class)
@Config(qualifiers = "w360dp-h640dp-xhdpi")
class FooScreenTest {
    @get:Rule val composeRule = createComposeRule()

    @Test fun renders_initial_state() { ... }
    @Test fun user_interaction_updates_state() { ... }
}

Goal is render + golden-path interactions, not pixel-perfect visual
regression. One or two tests per screen is plenty for the first pass.

Why this matters

Refactors and theme changes break Compose render paths in non-obvious
ways. Screen tests catch those without needing a human to spin up the
emulator and click through.