fix(desktop): raise default warm backend pool cap from 3 to 8 (#103401)

[kyssta-exe]

Fixes #103401

Five local profiles with the old 3-backend default saturated the hard spawn coordinator. The fourth profile waited 30s for a free slot while LRU eviction spared keepalive-fresh entries, surfacing Local backend start for "engineer" timed out while waiting for a free slot.

Raise the default maxBackends 3 -> 8 in both authoritative places: apps/desktop/electron/pool-limits.ts and its hand-mirrored renderer copy apps/desktop/src/store/pool-limits.ts. Persisted user values are untouched. At ~60MB per warm backend the new default (~480MB for 8) fits a realistic multi-bot roster without eviction; single-bot users still spawn exactly one. Mirrors the fix for #102822.

Test: vitest run electron/pool-limits.test.ts electron/pool-eviction.test.ts electron/pool-spawn-coordinator.test.ts — 140 passed (4 unrelated missing-electron fails). No other hardcoded defaults remain.

[alt-glitch]

This was generated by AI during triage.

Duplicate of #102828 — same two-file change (maxBackends 3 -> 8 in electron/pool-limits.ts and src/store/pool-limits.ts), which was opened first and also pins the new default with a test. This PR adds only a doc comment and a reference to #103401. Note that neither PR addresses the foreground-open starvation / relay spawn loop tracked in #102281 and #103375, which a larger cap only delays.

[kyssta-exe]

Closing as duplicate — triage AI alt-glitch identified this as Duplicate of #102828 (same two-file change maxBackends 3 -> 8). The duplicate label is present. Please follow the canonical PR #102828.