fix(lanes,git): harden the lane git surface and fix client races in the Changes panel

Server (git-diff.js, git-ops.js, worktree.js, routes/lanes.js):
- Every lane git call runs with GIT_LITERAL_PATHSPECS=1 and -z porcelain, so a
  file name is a file name, never a glob, and odd names parse correctly.
- A lane that is a subdirectory of its repo sees and acts on ONLY its own
  paths (reads, stage/unstage/discard/commit, and gitFacts.files).
- Any file named settings.local.json (the lane's router API key) is refused as
  403 ESECRET on disk (symlinks resolved) and at every committed rev.
- commit refuses with 400 ESTAGEDOUTSIDE while anything outside the lane is
  staged (checked with --no-renames, so a move into the lane is caught).
- push resolves its remote (upstream → only remote → origin), else 400
  ENOREMOTE; unstaging a rename unstages both halves.
- Git writes serialize on the repo's git dir, not the lane id, so two lanes on
  one index cannot collide on index.lock.
- git refusing an action (a numeric exit status) answers 409 EGIT with git's
  stderr instead of a 500 carrying a numeric code; one shared parseNumstat.

Client:
- Changes panel: no stale answer from a previous lane or an older quick-open
  query repaints the panel; discard disarms on lane switch; Enter on a file
  path opens it instead of folding; bounded tree expand-all.
- Large diffs (> 500 rows) render only the visible rows, with stable width
  and in-sync split columns; mirrored split scroll is clamped.
- Git bar: no "in sync" without counts, a clear no-remote message, branch list
  re-read after a switch, and no previous-lane facts after a lane switch.
- Tooltips no longer flicker or close on unrelated scrolls.

client/src/i18n/locales/en/lanes.json

@@ -25,13 +25,13 @@
   "addLaneBranchBusy": "{{branch}} — already checked out at {{path}}",
   "addLaneBranchExistingHint": "The worktree checks this branch out as it is — no new branch, no base.",
   "addLaneBranchExistingLabel": "Branch to check out",
-  "addLaneBranchHintAuto": "Leave empty to use the default name shown above.",
+  "addLaneBranchHintAuto": "Leave empty to let the server name the branch from the title.",
   "addLaneBranchLabel": "New branch name",
   "addLaneBranchModeCurrent": "Current branch",
   "addLaneBranchModeCurrentHint": "Work straight on this repo's checked-out branch (main) — no worktree, no fork. Switches to Repo mode.",
   "addLaneBranchModeExisting": "Existing branch",
   "addLaneBranchModeNew": "New branch",
-  "addLaneBranchPlaceholder": "feat/my-feature",
+  "addLaneBranchPlaceholder": "Automatic — named from the title",
   "addLaneNoBranches": "This repo has no commits yet — the worktree will start empty.",
   "addLaneNoFreeBranches": "Every branch in this repo is already checked out in a worktree. Create a new branch instead.",
   "addLaneExists": "This folder already has a lane: #{{id}} {{title}}. It is selected; pick another folder to add a new one.",
@@ -180,8 +180,10 @@
   "gitBar.commitStagedHint": "Commit only what is staged in the Changes panel",
   "gitBar.fetch": "Fetch",
   "gitBar.fetchHint": "git fetch --all --prune",
+  "gitBar.errNoRemote": "No remote to push to or pull from. Add one, or set this branch's upstream, in a terminal.",
   "gitBar.inSync": "in sync",
   "gitBar.newLane": "New lane here",
+  "gitBar.newLaneHeld": "Already checked out at {{path}} — git allows one worktree per branch",
   "gitBar.newLaneHint": "Open Add lane for a sibling worktree on the selected branch",
   "gitBar.noUpstream": "not pushed",
   "gitBar.pull": "Pull",