✨ feat: tui client estimated token cost (T28)

[Danathar]

Problem

The TUI token client can read token counts from GET /api/tokens, but it has no client method for the dashboard's separate GET /api/cost estimate. That leaves the Tokens pane without a source for its documented COST column: every per-agent and fleet cost remains unknown even when the dashboard has calculated an estimate.

The cost wire format also uses the numeric value 0 for both a genuine $0.00 estimate and an unpriced placeholder. Consumers therefore cannot infer whether a value is known from the number alone.

Implementation

• Add Client.Costs(ctx) and a narrow response envelope that decodes only the estimated portion of /api/cost through the shared authenticated getJSON path.
• Add CostSummary for total_usd, by_agent, and unpriced_models, plus CostAgentEntry for the complete per-agent token and USD breakdown.
• Preserve the server's source value and expose CostAgentEntry.Known() so callers use the source tag rather than guessing from USD.
• Add CostSummary.AllPriced() so a summary containing models without exact server-side prices cannot be presented as an authoritative fleet total.
• Add a full-shaped fixture that includes ignored gateway, per-model, per-session, metadata, and repository-count siblings. This proves the TUI projection stays deliberately narrower than the dashboard payload.

The client does not calculate prices locally; the dashboard remains the sole pricing authority. Known() accepts only source: "estimated", so an unknown or future source is handled conservatively.

Regression coverage

cost_test.go verifies:

• the authenticated GET /api/cost request and full fixture decode, including every requested per-agent field and all unpriced-model names;
• a real $0.00 value with source: "estimated" is known, while $0.00 with source: "unpriced" is unknown;
• a mixed priced/unpriced summary reports AllPriced() == false;
• a zero-usage response preserves valid, non-nil empty arrays;
• malformed JSON returns a decode error and a zero summary;
• non-2xx responses retain the existing typed *APIError behavior, including method, path, status, and response body.

Verification

From src/:

go test ./pkg/tui/client -run 'TestCosts' -count=1
ok  github.com/kubestellar/hive/pkg/tui/client

go test ./pkg/tui/...
ok  github.com/kubestellar/hive/pkg/tui
ok  github.com/kubestellar/hive/pkg/tui/client
ok  github.com/kubestellar/hive/pkg/tui/panes
ok  github.com/kubestellar/hive/pkg/tui/theme

go build ./...
PASS

The commands used task-local GOCACHE, GOTMPDIR, and CCACHE_DIR paths because this execution environment mounts their defaults read-only; no source or test behavior was altered.

Scope

This PR intentionally does not modify the Tokens pane, application refresh loop, native gateway balances, histories, session rows, repository counts, disclaimer, or price calculations. Rendering and joining cost rows to token rows remain T30 work.

Closes #5415

— hive: backend=codex

[kubestellar-prow]

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