fix(orbit): three interlocking guest playback bugs (#525)

* fix(orbit): guest short-circuits queue-exhaustion fallback paths

When a guest's local queue runs out (single-track queue from `syncToHost`
empties on `audio:ended`), the player walks the standard fallback chain
in `next()`: radio top-up → infinite-queue → stop. The infinite-queue
branch builds a 6-track queue and calls `playTrack`, which trips
`orbitBulkGuard` and pops a "Add 6 tracks to the Orbit queue?" modal.
Hitting Cancel leaves playback frozen; "Add them all" injects unrelated
tracks into the host's shared queue.

In an active Orbit guest session the host owns the queue. Skip the
fallback paths entirely and just stop — the next `useOrbitGuest` pull
tick will sync to whatever the host advanced to.

Bonus side-effect: kills the deferred-promise race where a
`buildInfiniteQueueCandidates().then(...)` from a guest's track end
could resolve *after* a Catch Up replaced the queue and pop the modal
a second time against the now-current 1-track queue.

* fix(orbit): treat natural track-end as not-diverged in guest sync

When a guest's track ended naturally before the host advanced, the
divergence-detection branch read `player.isPlaying === false` and
classified it as the user manually paused — so it refused to load the
host's next track. The guest sat silent until they clicked Catch Up.

`handleAudioEnded` keeps `currentTrack` pinned to the just-ended track
and resets `currentTime` to 0, while a real manual pause leaves
`currentTime` somewhere mid-track. Use the 0-position discriminator to
classify natural-end as not-diverged so the host's new track loads.

Confirmed via the captured guest log buffer:
  18:43:08.598 [track-change] host: VJkV5… → 6i6RP… BUT guest diverged
                              (player.isPlaying=false ≠ last.isPlaying=true)
  — guest stuck for ~33s until Catch Up was pressed.

* fix(orbit): Catch Up polls until engine is ready before seeking

The 400 ms blind setTimeout in `onCatchUp` was too short for an
HTTP-streamed cold-start on high-latency links. If the audio engine
wasn't ready by then, `seek(fraction)` silently no-oped and playback
started at 0:00, making Catch Up effectively useless on exactly the
slow links where it's needed. Captured log shows a Catch Up bringing
the guest to posSec=30, then 5 s later the guest was at posSec=6
(playback restarted from the head).

Replace with the same poll-until-ready pattern `syncToHost` already
uses: check every 100 ms, fire the seek as soon as the engine reports
playing, fall back to a blind apply at the 4 s deadline.

* docs(changelog): add orbit guest playback fixes entry

* fix(orbit): debounce Catch Up button + match bar item height

Two follow-on UX fixes after PR #525's three primary bugs landed:

1. **Debounce visibility.** Drift is computed from an asymmetric signal:
   guest's `currentTime` updates in coarse ~5 s chunks, while host's
   position is extrapolated linearly via `(nowMs - posAt)`. Even on a
   perfectly-synced session the diff swings ±5 s every tick, so the
   button flickered in and out continuously. Show only after drift has
   stayed over the 3 s threshold for ≥ 3 s of wall clock — measurement
   noise is filtered out, real sustained drift still surfaces in time.

2. **Match neighbour height.** The button was 32 px tall against 26 px
   for the other action buttons (.orbit-bar__settings) so every flicker
   shifted the entire bar height. Set `height: 26 px` and tighten the
   padding/font so the layout is stable regardless of visibility.

* fix(orbit): tighten queue-extension lockout + reliable initial-sync seek

Two follow-on fixes after the 4-bug umbrella:

**1. Local queue-extension paths fully off during Orbit.**
Phase check broadened from `active` to cover `starting` / `joining` /
`active` so a fetch-then-join race can't pop the bulk-add modal *after*
the join. The proactive infinite-queue topper inside `next()` (which
fires when ≤ 2 auto-tracks remain ahead) is now also gated, plus each
async `.then()` callback in the radio + infinite-queue paths re-checks
at resolution time. A `playTrack(... 6-track queue ...)` after the user
joined Orbit was the path that re-triggered the "Add 5 tracks?" modal
on a freshly-joined guest.

**2. `syncToHost` only seeks once the engine reports playing.**
The previous 2 s deadline-fallback applied the seek even when the
engine hadn't started, where the seek silently no-ops and the track
plays from 0:00. Symptom: clicking Catch Up makes the song "jump 50 %
forward" — that's the seek finally landing because the engine is now
ready, the initial-sync seek had already failed silently. New deadline
is 5 s, and on timeout we return `false` so the outer pull tick keeps
`lastAppliedRef` null and the 500 ms fast-poll retries.

* fix(orbit): double-click play button + hide preview during session

Two cucadmuh-flagged gaps:

**1. Double-click on the inline play button now reaches the orbit-add
   path.** The album-track row's onDoubleClick already routes to
   `addTrackToOrbit` when in Orbit, but the inline play button stopped
   propagation on click — so clicking it twice just fired the "double-
   click to add" hint toast and never touched the orbit queue. Add an
   onDoubleClick on the button itself that delegates to the parent's
   `onDoubleClickSong`.

**2. Track preview is suppressed during an Orbit session.** Preview
   shares the Rust audio engine with the shared playback, so starting
   one as a guest yanks the host's track out from under everyone. A new
   `[data-orbit-active]` attribute on `<html>` (set whenever role is
   host/guest and phase is starting/joining/active) hides every
   preview button via a single CSS rule, and `previewStore.startPreview`
   short-circuits as a defensive guard for keyboard shortcuts and any
   programmatic callers.

src/store/previewStore.ts

@@ -3,6 +3,7 @@ import { invoke } from '@tauri-apps/api/core';
 import { buildStreamUrl } from '../api/subsonic';
 import { usePlayerStore } from './playerStore';
 import { useAuthStore, type TrackPreviewLocation } from './authStore';
+import { useOrbitStore } from './orbitStore';
 
 /** Minimal track info needed to surface the preview in the player bar UI. */
 export interface PreviewingTrack {
@@ -75,6 +76,16 @@ export const usePreviewStore = create<PreviewState>((set, get) => ({
     if (!auth.trackPreviewsEnabled) return;
     if (!auth.trackPreviewLocations[location]) return;
 
+    // Block preview during any Orbit session — the preview path runs
+    // through the same Rust audio engine as the shared playback, and a
+    // preview started by a guest would yank the host's track out from
+    // under them. UI buttons are hidden via `[data-orbit-active]` CSS;
+    // this guards keyboard shortcuts / programmatic callers.
+    const orbit = useOrbitStore.getState();
+    const inOrbit = (orbit.role === 'host' || orbit.role === 'guest')
+      && (orbit.phase === 'active' || orbit.phase === 'joining' || orbit.phase === 'starting');
+    if (inOrbit) return;
+
     const current = get().previewingId;
     if (current === song.id) {
       await get().stopPreview();