Feature: add always recording/replay

[kreatoo]

I want to record all the time and save recording by keybind. This already exists in OBS but I prefer to use Cap so I would like it to be added if possible.

[linear]

CAP-692

[tembo]

This would be a significant feature addition. Here's some relevant context from the codebase:

Existing Recording Architecture

Cap already has segmented/fragmented recording infrastructure in crates/recording/src/fragmentation/ and crates/recording/src/output_pipeline/ that writes video in chunks rather than a single file. The FragmentManager (fragmentation/mod.rs) handles rotating segments with configurable duration. This could potentially be extended to maintain a rolling buffer of the last N minutes.

Hotkey Infrastructure

The hotkey system in apps/desktop/src-tauri/src/hotkeys.rs already supports custom actions including StartInstantRecording, StopRecording, etc. A "SaveReplayBuffer" action could be added following the same pattern.

Memory Considerations

Worth noting that #1589 is tracking high memory usage during extended recordings. The discussion there includes suggestions about chunking recordings into small segments that get finalized immediately - which aligns well with how a replay buffer would need to work. Any replay buffer implementation would need to carefully manage memory by discarding old segments as new ones are captured, and would likely benefit from fixes in that issue first.

Implementation Approach

A replay buffer would likely involve:

1. A new recording mode that continuously captures to a rolling window of segments
2. A mechanism to discard segments older than the configured duration (e.g., last 30s/1m/2m)
3. A new hotkey action to "save" the current buffer contents as a recording
4. UI to configure buffer duration in settings

The existing InstantRecording mode in crates/recording/src/instant_recording.rs could serve as a starting point since it already uses segmented output.