test: keep the client suite independent of the host locale

[miadisabelle]

Two client test files assert English output from code that formats in the host's default locale — correct for the app, but it fails the suite on a developer machine set to another language.

The two files on 3.1.0:

LC_ALL	before	after
fr_FR.UTF-8	3 failed	0
de_DE.UTF-8	4 failed	0
empty (Intl resolves und)	2 failed	0
en_US.UTF-8	0	0

SessionPicker

relativeTime formats with Intl.RelativeTimeFormat(undefined, …) and its tests match /day/ and /second/. Two days ago is 'avant-hier' under fr_FR, 'vorgestern' under de_DE, and '-2 d' under und.

The function already takes now so its tests are deterministic. It now takes an optional locale for the same reason, and the tests pass 'en'. The call site passes nothing, so the app still formats in the user's locale.

ContextMeter

The context and session titles use toLocaleString(), and the tests expected '50,000 of 200,000' and '12,500 tokens · 12,000 input' — '50.000' under de_DE, U+202F-grouped under fr_FR. The expected text is now built with toLocaleString(), as AnnotationMarker's and StatusDot's tests already do.

With both, npm run test:client passes 1150/1150 under fr_FR, de_DE and an empty LC_ALL.

If you'd prefer one line instead, setting test.env.LC_ALL in vitest.client.config.ts also works, since the forks pool starts its workers with that environment — but it pins every client test to English and depends on the pool staying process-based, so I went with fixing the tests.

typecheck, lint and format:check pass.

🤖 Generated with Claude Code

[miadisabelle]

Rebased onto 3.0.1 (f190ba0), no conflicts. Re-measured npm run test:client on the rebased branch: 930 passed under LC_ALL=fr_FR.UTF-8, de_DE.UTF-8, empty and en_US.UTF-8 — none of the client tests added since assume English.

[miadisabelle]

Rebased onto 3.1.0 (8c09c4c). The chat rebuild moved the context meter and its tests out of AgentChatView into chat/ContextMeter, and the new test hardcodes the same English grouping, so the fix moved with it. Re-measured on 3.1.0: the two files fail 3 / 4 / 2 under fr_FR / de_DE / empty before, 0 after, and the full client suite passes 1150 under all three.