feat: Volume Boost setting with system LoudnessEnhancer (#88) - #134

Adds a new Settings row "Volume Boost" under the Audio subsection that
cycles through 0 / 3 / 6 / 9 / 12 / 15 dB. 0 dB is the default and
attaches no effect. Above 0 dB, the player creates an Android
`android.media.audiofx.LoudnessEnhancer` bound to the ExoPlayer audio
session and applies the target gain. Useful for content whose source
audio is very quiet relative to the user's TV/speaker setup \u2014
repeatedly requested in #88 and comparable to the volume boost feature
in Debrify TV that the OP referenced.

Changes:

- `SettingsUiState` / `SettingsViewModel`: new `volumeBoostDb: Int`
  field. Stored as a string in DataStore via
  `profileManager.profileStringKey("volume_boost_db")` because
  ProfileManager has no int helper. Parsed back to Int on read with a
  0-15 clamp. `cycleVolumeBoost()` mutator advances through the steps
  and triggers cloud sync.
- `SettingsScreen`: new `Audio` subsection at the bottom of General
  (below `Network`) containing a single `SettingsRow` for "Volume Boost".
  The row is placed at `focusedIndex == 14` so no existing indices
  shift. Both the auto-scroll max-index and the D-pad-down max-index
  clamps are bumped from 13 to 14. The Enter handler maps
  `14 -> viewModel.cycleVolumeBoost()`. DNS Provider stays at index 13.
- `PlayerUiState` / `PlayerViewModel`: new `volumeBoostDb: Int` field,
  loaded from the same DataStore key during `loadDetails` initialization.
- `PlayerScreen`: new `DisposableEffect(uiState.volumeBoostDb,
  exoPlayer.audioSessionId)` that creates a `LoudnessEnhancer` when
  targetDb > 0 and the session id is valid, sets target gain in millibels
  (dB * 100), enables the effect, and releases it on dispose. Wrapped in
  try/catch because some Android TV devices reject audio-session effects
  when HDMI passthrough is enabled for DTS/AC3 \u2014 we fail silently and
  the user gets unboosted audio but playback still works.
- `CloudSyncRepository`: `volumeBoostDb` added to `CloudProfileSettings`,
  `volumeBoostDbKeyFor(profileId)` helper, push/pull wiring so the
  setting syncs across devices. Default is 0 so existing users see no
  change. Stored as string via profileStringKeyFor for the same reason
  as the SettingsViewModel side.

Cap: +15 dB (1500 millibels). Higher values tend to introduce audible
distortion on already-compressed streaming audio. The LoudnessEnhancer
class supports more but we intentionally don't expose it.

This PR touches `SettingsScreen.kt`, `SettingsViewModel.kt`, and
`CloudSyncRepository.kt` which PR #131 (Hide Budget, #72) also
modifies. Both PRs add a new settings row with different indices and
different fields in `CloudProfileSettings`. Whoever merges second will
need a small rebase to resolve:
- General section max-index should become 15 items (max 15) with both
  rows present.
- `CloudProfileSettings` needs both `showBudget` and `volumeBoostDb`
  fields.
- The Enter handler needs both `13 -> cycleVolumeBoost` and
  `14 -> openDnsProviderPicker` if #131 lands first, or an additional
  `15 -> setShowBudget` entry if this PR lands first.

The conflict is purely additive and both PRs can coexist on main.

Closes #88