Diagnostics: opt-in debug capture with one-click copy from Preferences

[daiverd]

Summary

• Adds src/diagnostics/, a bounded, in-memory-only ring buffer (DiagnosticsRingBuffer) capped by both record count and a rough byte estimate. record(category, data) is a zero-allocation no-op while disabled, so it stays cheap to call from hot paths.
• While capture is enabled, records: console.warn/console.error (wrapped once, original behavior preserved, message capped to 500 chars), connection lifecycle transitions + reconnect count (via useConnectionStore), and counters sampled every 5s (inbound messages/sec and output lines/sec, from useChannelHistoryStore/useOutputStore entry-id deltas — no per-event hooks) plus long-task count/duration via PerformanceObserver.
• Adds a Diagnostics tab to Preferences with an off-by-default Capture diagnostics checkbox, a redact message text toggle, and a Copy diagnostics button. The pane states clearly — before copying — that the export includes character name, room names, and possibly message text, and never includes passwords/tokens/credentials.
• getDiagnosticsMarkdown()/copyDiagnosticsToClipboard() assemble an environment + session snapshot (UA, platform, hardwareConcurrency, deviceMemory, window size, devicePixelRatio, page uptime, app version from package.json, connection status, live subsystems — audio via client.media.cacophony.context.state, MIDI enabled pref, editors open, file transfers) plus the buffer contents, and serialize everything to Markdown ready to paste into a GitHub issue.
• Added small openEditorCount/activeTransferCount getters to EditorManager/FileTransferManager so the environment snapshot can report live subsystem state without reaching into private internals.
• The capture toggle and redact-toggle live in preferencesStore (persisted like other preferences); the ring buffer contents themselves are never persisted to localStorage/IndexedDB, per the issue's privacy requirements.
• record(category, data) is intentionally generic/decoupled from any specific producer, so the performance watchdog (issue Performance watchdog: detect and attribute sustained main-thread work while idle #103, built in parallel) can call it once both land — this PR has no dependency on or reference to watchdog code.

Test plan

• npx vitest run — 113 test files / 1101 tests passing, including 7 new colocated test files under src/diagnostics/ covering ring buffer bounding (record count + byte estimate) and FIFO eviction, disable = no-op (record count and JSON.stringify spy), console capture wrapping + message capping, connection lifecycle + reconnect counting, counter/long-task sampling, environment snapshot assembly, and Markdown shape + redaction behavior.
• npm run typecheck — clean
• npx biome lint on new/changed files — clean (two pre-existing, unrelated warnings in App.tsx predate this change)

Fixes #100

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