fix(desktop): preserve commit reword drafts across navigation

[ameyypawar]

Reword (edit commit message) drafts were lost on context switch — start editing a commit's message, navigate to another commit/branch/repo, come back, and the draft is gone. New-commit drafts already survive.

Cause: CommitView.svelte mounted the reword editor seeded from the original message with no onChange, so the draft lived only in the editor's local state and was destroyed when the editor unmounted on context switch. NewCommitView avoids this by persisting into lane.newCommitMessage (redux).

Fix: mirror that, scoped to reword — persist the draft per-commit in lane UI state, seed the editor from it, clear it on save or explicit cancel:

• lib/commits/editCommitDraft.ts — pure resolveEditDraft / applyEditDraftUpdate helpers (+ EditCommitMessage type), with unit tests.
• lib/state/uiState.svelte.ts — add editCommitMessage to lane StackState.
• components/commit/CommitView.svelte — seed from the draft, persist on change, clear on save + explicit cancel.

Behavior: navigate away / close the drawer → draft preserved; return → restored; Cancel → discarded; Save → cleared. (RichTextEditor only seeds when empty, so restore happens on remount and there's no caret reset while typing.)

Two design points worth a maintainer call (cc @PavelLaptev, per the issue thread):

1. Editor Cancel/Escape discard, but Drawer close / navigation preserve — biased toward preserving so navigation reliably keeps the draft. Happy to make Drawer-close discard too if preferred.
2. One reword draft per lane, keyed by commit — starting to edit a second commit before returning overwrites the first's draft.

prettier, eslint, svelte-check (0 errors / 0 warnings), and the new vitest suite (7 cases) pass locally.

Fixes #13287

[Byron]

Thanks for giving it a shot.

Could you please add before and after videos? This would make it so much easier to get started with the review.
Thanks again.

[ameyypawar]

after.mov

[ameyypawar]

before.mov

[Byron]

Thanks a lot for the videos!

In the interest of time, I didn't validate this locally, just because I think the PR should receive its final approval from @mtsgrd. To me the code looks good and quite minimal, but I am missing the big picture to truly judge its fitness.