fix(desktop): non-blocking rebase conflict handoff + word-diff freeze guard

[devlint]

Summary

Fixes #128 — two of the three reported issues; the third (MergeEditor "clunky" UI) is intentionally scoped out, see below.

• Non-blocking conflict handoff: when an interactive rebase pauses on a genuine merge conflict, RebaseEditor now closes its blocking modal and hands off to the existing non-blocking rebase banner + inline MergeEditor, instead of trapping the user behind an overlay with no resolution UI. edit/split halts (not a conflict) are unaffected — the modal still opens for those, with its "Split this commit…" affordance intact.
• Rebase-aware finalize: resolving the last conflicted file mid-rebase no longer triggers a spurious git merge --continue error; the rebase banner's own Continue button is now the sole finalize path during a rebase (this also fixes a pre-existing latent bug affecting plain pull --rebase conflicts).
• Freeze fix: diffHighlight.ts's word-level LCS highlighting is unguarded O(m·n) — a conflicting line with thousands of tokens (minified bundle, long JSON, base64) could blow up the DP table and freeze the UI on every resolution click. Added a WORD_DIFF_MAX_CELLS = 1,000,000 threshold guard that falls back to whole-line highlighting above it, mirroring the guard already used in packages/core/src/diff/lcs.ts for a different LCS shape.

Scoped out: issue #128 also described the resolution UI (MergeEditor.vue) as generally "clunky." After review there's no single, cheap, unambiguously-broken affordance to fix there — it's a diffuse complaint about several overlapping resolution mechanisms. A redesign belongs in its own chantier, not bundled into this bug-fix PR.

Known limitation (documented in ROADMAP.md): after this handoff closes the modal on a conflict, a later edit/split halt within the same rebase won't re-surface the "Split this commit…" affordance — only the banner's Continue/Skip/Abort. Still strictly better than the prior always-blocking modal; tracked as a follow-up.

Test plan

• pnpm test — 84 files / 651 tests pass (includes new diffHighlight.test.ts regression test for the freeze fix)
• vue-tsc --noEmit — clean
• pnpm test:parity — pre-existing unrelated failure only (scan-secrets.test.mjs, stale prebuilt Rust binary predating the secrets scanner; reproduces identically on main, confirmed via stash A/B)
• Manual QA in a real browser (pnpm dev:web, real git backend, no mocking) against throwaway temp repos:
  • Conflict halt closes the modal, shows the rebase banner, opens MergeEditor inline — nothing blocked
  • Resolving + banner Continue finishes the rebase cleanly, no merge-continue error (verified via git status/git log)
  • edit/split halt (no conflict) still keeps the modal open with Split intact — no regression
  • A ~39 KB / 5000-token conflicting line falls back to whole-line highlighting and stays responsive (~7-8ms frame time, vs. the prior freeze)
  • Probed: aborting mid-handoff recovers cleanly; a second conflict later in the same rebase is handled by the banner (no modal left to trap it)
• A real-git-repo automated test for the useInteractiveRebase conflict signal was attempted but destabilized the parallel test suite (confirmed via A/B against main) without adding real coverage beyond the manual QA above — dropped rather than left flaky or backed by a mocked git layer.