…client

Plain `throw new Error(...)` was sanitized by oRPC to a generic
"Internal server error" on the client, hiding the actionable
`git fetch origin && git remote set-head origin --auto` suggestion
resolveBase() tries to surface. Throw `ORPCError("PRECONDITION_FAILED",
{message})` instead — oRPC passes defined errors through with their
message intact.

Also fix the "Mode toggle defaults to Local" scenario: the test
terminal boots at `~` (not a git repo), so the toggle wasn't rendered
at all. Seed a throwaway git repo like the other Review-tab scenarios
already do.

The right-panel tab gets a more honest name: "Code Diff" instead of
"Review". Two reasons:

1. The tab does one thing (show changed files + their unified diff);
   "Review" oversold a workflow that doesn't exist yet (no inline
   comments, no agent handoff — those are later phases).
2. "Review" is forge-coloured (suggests GitHub PR review). "Code
   Diff" is forge-agnostic and stays accurate if Bitbucket/GitLab
   support lands later, or if non-Git VCS support ever lands.

Schema enum: "review" → "diff" (short, single token). Tab label:
"Code Diff" (display only). Migration `1.9.0` maps existing
`"review"` persisted values forward; the historical 1.8.0 migration
keeps casting the stale enum through `string` so old codepaths still
compile under the live `RightPanelTab` type.

File renames: ReviewTab.tsx → DiffTab.tsx, review-tab.css →
diff-tab.css, review_tab_steps.ts → diff_tab_steps.ts, review-tab.feature
→ diff-tab.feature. All `review-*` testids become `diff-*`; the diff
content area is `diff-content` (avoiding the awkward `diff-diff`).

Brings in #523 (KOLU_STATE_SUFFIX crash fix). No conflicts; the
state.ts changes are independent from this branch's RightPanelTab
schema migration.

"Code Diff" → "Code" — the broader name leaves room for a future
file-tree-browser mode under the same tab. The cycle-on-click label
is replaced by icon sub-tabs (pencil for local, branch-fork for
branch diff) that naturally extend to a third mode without rethinking
the header layout. A muted mono context label (e.g. "vs origin/master")
sits after the icons so the user still knows which base they're
diffing against.

New DiffLocalIcon and DiffBranchIcon added to Icons.tsx.

The tab is called "Code" (not "Code Diff" anymore) — the component
and CSS file should match. A future file-tree-browser mode will live
under the same tab alongside the two diff modes.

Covers renames (R100), copies (C075), type changes (T), unknown
letters (→ '?'), empty input, blank-line tolerance, and sort order.
The parser is hand-rolled from git's --name-status output and these
edge cases aren't reachable via e2e.

Exports parseNameStatus (was private) so the test can import it
directly.