Fix timestamp message search hanging on empty partitions and 'topic undefined' 404 on navigation

[jvorcak]

Fixes two customer-reported issues (Zendesk ticket 7199).

1. Backend: `[unknown] request was cancelled while waiting for messages` with Timestamp start offset

A start offset resolved by timestamp is `-1` on an empty partition. `calculateConsumeRequests` clamped it to `highWaterMark - 1`, which is `-1` again for an empty partition — and kgo interprets `At(-1)` as "start at the end", i.e. an indefinite live tail. The round-robin allocator also assigned a message to such partitions, so the request could never be satisfied and only ended via cancellation after the server deadline (~35s). One empty partition in an otherwise populated topic was enough to make every timestamp search fail this way.

Fix: exclude partitions with nothing to consume (`high <= low`) from the consume request (except deliberate live tail), and complete immediately — instead of returning an error — when all partitions are empty.

Verified live via `buf curl` against the streaming RPC:

• empty 3-partition topic + timestamp offset: before → hang + cancellation error after 35s; after → `done` in 2ms
• topic with data in one of three partitions: before → messages returned but request still dies after 35s; after → messages returned, `done` in 7ms

New unit tests: `TestCalculateConsumeRequests_EmptyPartitions`, `TestCalculateConsumeRequests_AllPartitionsEmpty` (live tail still includes empty partitions).

2. Frontend: 404 "The topic 'undefined' does not exist" when navigating away mid-fetch

nuqs flushes queued URL updates inside `startTransition(() => navigate({ to, from }))`. Frequent synchronous re-renders during an in-flight message search ("Fetching data...") keep preempting that transition; when the user clicks away, their navigation commits first and the deferred flush then runs `navigate` with a `from` route that is no longer matched. TanStack Router interpolates the missing `$topicName` param as the literal string `undefined`, navigating to `/topics/undefined` — a persistent 404 (console warning: `Could not find match for from: /topics/$topicName/`).

Fix: wrap the tanstack-router nuqs adapter (`utils/nuqs-tanstack-adapter.tsx`) so a flush is dropped when its originating route is no longer in the router's pending/current matches, checked at flush time.

Verified with a headless-browser repro (empty topic, start a hanging search, click "Topics" while "Fetching data..." shows): before → `/topics/undefined` + persistent 404, 3/3 runs; after → lands on `/topics/` normally.

Repro (for QA)

1. `rpk topic create repro-empty -p 3`
2. Topic → Messages tab → Start Offset → Timestamp (any time) → search errors after ~35s (issue 1)
3. While "Fetching data..." shows (use Latest / Live on the empty topic for a stable window), click Topics in the sidebar → URL becomes `/topics/undefined` (issue 2)

Testing

• `go test ./pkg/console/`, `go vet`, `gofmt` clean
• frontend `type:check`, `lint`, full `bun run test` (1259 tests) pass

[github-actions]

✅ Clean — no registry drift, off-token colours, or ad-hoc classes

App: frontend · Scope: diff vs origin/master · Files: 3

	Count
⚠️ Outdated registry components	0
🛠 Locally-modified components	0
❓ Unknown to registry	0
🎨 Off-token palette colours	0
🔢 Ad-hoc utility classes	0

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