Feat/discord hide when paused

[SyahrulBhudiF]

What and why

Closes #63.

Adds Hide status when paused under Settings → Account → Discord. It is disabled by default, preserving existing paused presence behavior. When enabled, pausing clears Discord Rich Presence and resuming republishes the current track. Presence synchronization is serialized so a delayed clear cannot overwrite a later resume update. This PR also fixes the shared segmented-tab label treatment: mix-blend-exclusion made selected labels turn cyan/green on Zuno's red accent and the hover opacity transition flickered.

How it was checked

• npm run verify passes — 31 checks passed
• cargo test not required — no src-tauri/ files changed
• Ran the app, enabled the Discord pause setting, paused and resumed playback
• Screenshot below (UI changes)
• npm run build passes

Screenshots

Discord settings

The setting is in Settings → Account → Discord and is explicitly separate from the master presence switch.

Notes for the reviewer

• The toggle defaults to off, as requested in Add toggle to hide Discord Rich Presence when music is paused #63.
• DiscordRpcService owns the playback-to-presence policy; PlayerController supplies only the current playback snapshot.
• The presence synchronizer deduplicates repeated commands, retries failed transport operations on the next sync, and serializes clear/update IPC so pause/resume cannot finish out of order.
• src/player/DiscordRPC.check.ts covers pause clearing, resume republishing, immediate toggle changes while paused, in-flight clear/resume ordering, and retry after a failed clear.
• The segmented-tab fix is isolated in its own commit: e7fa83e.

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